I'd like to play the Sims 2 Season in windowed mode so I can still IM and e-mail while I'm playing.

I've tired both single and double clicking while holding down either Command or Option, or both at the same time. Neither brings up the preferences dialog. Any ideas?

Try this:
http://forums.macrumors.com/showthread.php?t=333615

ed: sorry, you tried this. Might be a Leopard bug, try reporting it.