This looks like a really cool app, but I'd rather see it released as something you could install on your own server, like phpMyAdmin. It looks like it runs from their website and accesses yours via FTP, which I'm not really that comfortable with.
We thought seriously about turning AppJet into this, before we decided EtherPad was a better business. We even built version 1 of a virtualized php-hosting backend. In the end, we just couldn't get our hearts into it because we liked programming in JavaScript so much better than PHP.
Neat concept, though as other commenters have pointed out there are some security issues.<p>On the plus side, I can't imagine it being that hard to add syntax highlighting for additional languages.