Will the darwin opensource bit have backward compatibility?

Future prognostication is prohibited on these forums. Please read the terms of use. Your question requires someone to have a crystal ball.

As for whether you can install the current Darwin on Mac OS X 10.4, see if anyone on the Mac OS X Technologies Unix forum has tried that as it is a part of Mac OS X's Unix kernel:

http://discussions.apple.com/forum.jspa?forumID=735

You do not install Darwin "onto" Tiger or any other version of Mac OS X. Darwin is the open-source equivalent of the kernel portions of Mac OS X - specifically the kernel, device drivers and driver support frameworks, a BSD personality layer, and various libraries and command-line utilities - and so would replace Mac OS X on any given system. Darwin 9.2 is still available for PowerPC systems as was as Intel systems. But how long that will continue to be the case for future releases is unknown, though most likely should some future version of Mac OS X no longer supports PowerPC, were that to happen, it's associated Darwin components would quite likely follow suit.

It will work just fine, but not as part of Mac OS X. Apple doesn't provide all the interface code between the Darwin layers and the rest of Mac OS X (Aqua, etc.) as part of the Darwin package, and almost without doubt such "glue" code changes from version to version, so it would almost certainly not be possible to yank out the Darwin "guts" of Mac OS X and replace it with a different version of Darwin.

If you boot Mac OS X in Single User Mode (command-S) you'll see a version of Darwin is loaded regardless of whether you've installed the Developer Tools.

Mac OS X continues Apple's path of tight software/hardware integration, and Darwin kernel is at the core of Mac OS X if you read the documentation here:

http://en.wikipedia.org/wiki/Darwin%28operatingsystem%29

I would not attempt to load one Darwin microkernel on top of another, unless I knew I wasn't about to create a situation where you might get a kernel panic because of unexpected commands being passed to the kernel. Given the fact that software itself that is poorly developed can create a kernel panic when it is designed as kernel software, modifying the entire Mac OS X microkernel avoiding the existing microkernel suggests the operating system itself may not be stable enough to accept an entirely different microkernel. If there are exceptions, the people in the Unix forum would know.

You probably could install pure Darwin on your iBook. It runs on PCs, so it should run on an iBook. In this sense, Darwin is going to be no different than FreeBSD or Linux. It will not be MacOS X. You would have to run X Windows on it. Unless you really enjoy hacking around on an OS, it would probably be much easier to get a new Macbook.

I'm still not convinced

I think most people familiar with OS X would probably assume it would be total folly to try and install a later version of Darwin into an earlier version of OS X. But that could be totally wrong. Why don't you give it a try and and let us know how it goes?

Yes, I generally have some idea what I'm talking about, though I won't claim that I'm never wrong. I really don't care if you're convinced or not. If you don't believe me, feel free at any time to compile and attempt to install the Darwin components in replacement for the components included with Mac OS X. I've already explained why it probably won't be possible, but maybe it would work for you, though I really doubt that it will. Just don't come whining here and worse blaming Apple or anyone here if you break your installation of Mac OS X and have to completely reinstall.

'nuff said. I've given you my (reasonably informed, I think) opinion. It's up to you from there.