Could be a failed hard drive. Do you have a backup?
Will it boot in Safe Mode? See What is Safe Boot, Safe Mode? (Mac OS X)
http://docs.info.apple.com/article.html?artnum=107392 Takes a while to run, but it usually "fixes" problems.
Resolve startup issues and perform disk maintenance with Disk Utility and fsck
Will your Mac boot using the system install DVD (or another system CD/DVD)?
Here's what finally happened, if anyone's interested.
Doing the Apple Hardware Test didn't work, it kept stalling partway thru.
Was FINALLY able to startup from the install disk holding C key (it kept ejecting the disk several times) and did a Repair to the Hard Disk using the Disk Utility, and that seemed to do the trick.
This is all the same as in this other post: