I think psd versioning is outside of github's core competency. We've used distributed version control for design files, and it really doesnt support a natural workflow. Background syncing just seems much more natural as a solution to the problem since the files tend to be very large, and are compounded by rapid generation and disposal of many many versions.
Another issue that might be how integrated the version control system could be into the graphics app. Github will never aim to integrate itself deeply into Photoshop, so designers would be forced to visit a browser to see changes.
Something like OS X Lion's file versioning system might actually be perfect for this.
