I am making a small game number game in Java (Nim) in OS X and was thinking about trying to make the GUI interface of the game using something other than Swing. I looked at
You might consider JavaFX 2.0.
Still early days but it looks quite promising for the kind of interactive / graphical applications that you might otherwise use Flash for.
It depends, with eclipse 3.4, SWT 3.4 is quite supported with MacOs.
Now, SWT is OS specific, and you may not have the same flexibility than Swing, so you need to have good reason for looking for an alternative to Swing, especially when you consider there are good LAFs (look and feel) for java.