Cannot eject from Finder (Lion)
If this behavior persists past a reboot, you might need to go down some general problem isolation steps:
- Isolating issues in Mac OS X
There used to be an article (it's gone or I can't find it at the moment) that walked you through setting aside the application preferences or testing this behavior in a new user. In your case - it looks like you figured it out, but I wanted to at least mention the official guide in case others have the same symptoms but a different cause.