It supports anything that QuickTime can play, some formats better than others. But I use it to edit DiVX. Note, that it will always save a quicktime movie, even if you are editing an AVI or MPEG, but it won't re-encode, it just changes containers. Email me jon@interfacelab.com and I'll hook you up with the latest version that supports AVCHD.
This bummed me out, not only for Shave, but as a user. But, you know, Apple's direction with media is a little f'd in my estimation. AVFoundation is dope, but it's a huge step back from QuickTime. Shave 2 is written using it, and so far (and it has a long ways to go), it's not half what the QuickTime version is.
(Sorry if I missed the answers on your website: I browsed through and didn't find them, but that might be my fault.)