Since Steve didn’t turn on comments for his post here, I’ll speculate over in my neck of the woods.
The main question is, why won’t time machine simply back up to a standard file share? If it’s HFS+ shared over AFP, what’s the difference between that and locally attached storage?
Well my guess is this: File sharing protocols do not equal file systems. A variety of filesystems can be shared over AFP without OS X knowing about it. NFS shares can be re-shared over AFP putting 2 protocols between the OS and the file system it wants to write to.
Time machine is probably wanting to preserve a whole lot of fun metadata that require special attributes of the file system. If you can’t guarantee that you’re writing to HFS+, you might have data being eaten.
One way to guarantee that you’re writing to HFS plus is to have a daemon running on the other computer that has direct access to the file system, and makes sure things are getting written the way they should. Something AFP doesn’t really do.
Could they patch AFP to make it serve the purpose? Possibly, but it would probably be a pain in the ass, and you’d have to back port it to 10.3 and 10.4 and who wants to do that?
Best bet? Buy a Mac Mini when leopard comes out, with the OS pre-installed, and use USB enclosures. They are ass-cheap and will serve your purpose. Or buy a used intel mac mini for even cheaper, and throw leopard on it when it ships. Or, just keep your current setup and use rsync for your backups.