The functionality and usefulness of this service is fantastic, but I wonder how hard it would be for github itself to compete; they've shown that they can do simpler binary versioning and comparison with PNG/JPEG images, so all they'd have to do is add support for previewing PSD files as well. Whether that's a simple thing to support is not something I'm qualified to answer though :)
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.