Clever of them to show a custom welcome message for Hacker News visitors. I love how it jumps you right into a demo app with no login or registration steps.
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 :)
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.
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 :)