O.K., folks,
I was going to update this post anyway, since it's no longer true, and since SSGT. BARNES brought it up, now's the time:
So, as a 64-bit file format that can store a huge amount of Audio, as well as many other types of information, it's not a file type that GarageBand would recognize or be able to use.
Well, it was true when I wrote it, but it's not now.
The new Leopard OS is going to be optimized for 64 bits.
It's obvious that GB '08 is placed to take advantage of that fact (as I'm sure the updated Logic will be, when it finally comes out).
So, with GB '08, .caf files are used. One example is the new .wand files that are used in "Magic GB". They contain .caf files that contain, among other things, the audio files.
WH