Some keys not working

Booted from the OS X install DVD, you could try and repair disk, then
also repair disk permissions (using Disk Utility in the booted Installer
menu options) or just try restarting the computer with the Shift key
held down, until the Mac starts; keep holding it until you see a login
and you enter your account password; then find Disk Utility after
the Finder appears normal; run 'repair disk permissions' then quit
Disk Utility, restart the computer normally, and see what happens.

• Mac OS X: Starting up in Safe(boot) mode:
http://support.apple.com/kb/ht1455

This may 'fix' the issue. Or it may be some hardware or software is bad.

If you can locate and use an Apple USB keyboard (white plastic version
should just work; a later metal version may not just work w/o drivers) it
may be of help to troubleshoot the computer's other hardware. Could be
the iBook's keyboard may be bad, or there may be another item at fault.

Or it could be some settings are no longer working on a default setting.

{The default effect a click on F12 may be expected to do, could be changed.
You can change or check the effect via the Keyboard preference panel.
There also may be a second location to see about assigning functions.}

If you activate Keyboard Viewer (from International panel in System Pref)
you could see if there is an interaction from clicking on single keys in the
Viewer. You can type via mouse or trackpad this way, too; or test to see
if the keyboard itself is really registering key-taps. It also shows the other
alternative key functions in the Viewer window.

Could be the internal keyboard is acting up; or maybe there is another
different issue involving hardware or software. Corruption to a system
could occur if the hard disk drive has bad or damaged sectors; or if
there is a problem otherwise in the computer.

You could try a new installation and when booted from the OS X 10
install DVD, choose the Disk Utility from the Installer's menu bar,
and in Disk Utility, you can choose to run Secure Erase. This will
give you one-pass over-write of zeros over the hard drive contents.
A feature to choose a partition map (Apple Partition Map on PPC)
is required, then choose to format the hard disk drive; use HFS+.
Then, the hard disk drive should have plenty of free space in one
large block, no old files would be readable; and no fragmentation.
Any small hard disk errors would be overwritten or ignored.

A reset of the Power Management Unit may have some merit, but
that is often an item of last resort. The iBook model instructions on
how to do the PMU reset vary among build model/year.

• Resetting PowerBook and iBook Power Management Unit (PMU)
http://docs.info.apple.com/article.html?artnum=14449

• How to reset the PRAM and NVRAM:
http://docs.info.apple.com/article.html?artnum=2238

{In some models, the PRAM is reset when the PMU is reset. Both
do different things; so read into them before launching either one.}

• One or more keys on keyboard do not respond:
http://support.apple.com/kb/ts1381

• About Keyboard Viewer (OS X 10.4.x etc)
http://docs.info.apple.com/article.html?path=mac/10.4/en/mh2325.html
(You can have Keyboard Viewer accessible from Finder menu bar.)

• Mac OS X keyboard shortcuts:
http://support.apple.com/kb/ht1343

If the Keyboard Viewer shows you are indeed typing, but nothing
else is happening, then there could be some kind of software issue.
And, if you can type with a mouse or trackpad click, but not with a
keyboard, then the keyboard (internal to iBook) may be bad.

The entire keyboard can be removed from the portable Mac and a
replacement (new or used/good) could be obtained to fix a bad key.

To have a suitable external USB keyboard and mouse on hand, so
as to facilitate troubleshooting and/or continue using a Mac if or when
the original keyboard acts up, is a good idea. I try to find deals ahead
of any indication of need; I like the comfort of a new extra spare parts.
Spare running computers is also a fine idea, esp. when away from town.

Good luck & happy computing! 🙂

+{ edited to add links, etc }+

The EXACT same keys have just stopped working in my G4 keyboard-- I'll try the various suggestions. I really don't want to go to an external keyboard (sort of defeats the whole laptop concept). I think something is up with software because that's just too much of a coincidence.

Ok, typing with the mouse on the keyboard viewer gives results for those keys, but they do not register when typing on the actual keyboard. So the problem is the keyboard itself, then? And a replacement laptop keyboard should fix it?

Probably a replacement keyboard of correct spec for your computer
model should work and solve the problem. The basic info on how to
remove/replace a keyboard should be available online, for free.

Guides for portable computers are at iFixit.com & there also are some
download .pdf documents on how to upgrade RAM and replace a
keyboard for some of the iBook G4 models, hidden in Apple Support
database in the Apple site. Probably via the iBook hardware link in
the left margin of the Support page, where other products are listed.
It would require some looking. The iFixit site may be faster, depends
on what model and build year your machine, and get a matching fixit.

• iFixit Guides: http://www.ifixit.com/Guide/Mac/
• iBook G4 fixit: http://www.ifixit.com/Browse/iBook_G4

• Apple - Support - iBook (portable computer)
http://www.apple.com/support/ibook/

If your iBook G4 portable computer is a 14" 1.42GHz model
and considered a Mid-2005 model, you can get a download
of some interest (from a link found in Support database) here:
http://manuals.info.apple.com/enUS/iBook_G4_Mid_2005_UsersGuide.pdf

If the above model is what you have, be certain of the correct
replacement keyboard part number (and of a good return policy)
before buying one for your iBook G4 1.42GHz or whatever.

And if you have the 14" iBook G4 (mid-2005) the keyboard part
number should be: *keyboard part number: 922-6913*. Even so,
it is advised to be certain because some look alike but aren't.

You may find an Authorized Apple location with certified techs
and they may have some pre-owned and working keyboards.
If so, then be sure whatever you get (wherever) is correct and
the seller has a decent return/exchange policy.

There are online sources for parts, real store-like places, and
some who sell stuff via auction or ebay craigslist. Be wary if
you can't see the keyboard and actually make use of it directly.

The choice of a matching Guide to remove/replace the keyboard,
is available from the main page where you choose your G4 model.
Some iBook G4 and G3 white models are different, though they
do look somewhat similar. Changes occurred over the build years.

You may be able to find a fair deal on an as-new or previously used
keyboard from a computer that had a different problem, so check
into the price range and don't fall some escalating auction rip-off.
Some of them vary in price from $20. to $50. and each series of
iBook G4 (12" & 14", too) have different keyboard part numbers.

Hopefully this helps somewhat.
Good luck & happy computing! 🙂

+{ edited to add links as I found them }+

Thanks-- those are some helpful tips. Looks like I'll hunt for a replacement.