Unity 3 suddenly won't start (OSX 10.6)

Using Unity 3 on OSX 10.6; was working fine, now suddenly won't launch. Launching the app gets no feedback. Unity icon appears in the Alt+Tab switcher but switching to it doesn't do anything, application menu remains unchanged (e.g. still shows Finder menu). top shows the process is running.

Launching from the terminal (/Application/Unity.app/Contents/MacOS/Unity) gets a spinning beach ball but nothing else, no errors.

I deleted the plists, but that didn't help. No errors in the Console or logs, either.

Maybe it's the project? Hold down option while launching Unity, and choose a different project. There's an option to show the project wizard at startup in the preferences, but of course you'd need to actually be able to launch Unity first.