"for Mac OS X" breaks my heart, because this is exactly the kind of open-source DB client I want on Linux. At least I can use this when I'm on my Air, but I recently went looking for a multi-DB client app like this for Linux, and this looks like what I was wishing I would find.
Written to what API? Gnome, X, Qt? You'll be addressing a slice of a fraction of potential users. It'd be great if it weren't the case, but the diversity that makes linux interesting to hackers also makes it really tough to roll out GUI apps like this and have it 'just work'. I get the sense that's a good part of the appeal of OS X for the developer community lately - mostly Linux-ish when you need it, but a unified…
Both GTK and QT are available on OS X, Windows, and Linux. Cocoa is available on OS X only.