MAME OS X works like a charm under Leopard, here's the proof:

With Spaces(TM) you can even move your game window in a different space from the browser window, and Leo will switch from a space to another one automatically when you launch a game or when you exit from a game.
The only drawback is that since the CTRL key is used along with the arrow keys to switch between spaces, when you will play a game with the keyboard you will frequently switch accidentaly the actual space...