I've heard bad things about CS3 and Snow Leopard - maybe not a wise purchase if you use a Mac. Actually, I've heard sporadic reports of CS3 breaking starting a few updates ago...
(After checking into this I find that, while CS3 is not officially supported on SL, it mostly works OK, and has had some testing by both Adobe and Apple.)
My copy of Snow Leopard arrived today, so I'll let you know what I find. I'll be upgrading a 2009 Mac Mini 2ghz/2gb (home machine), and a 2008 Mac Pro 8 core 20 gb ram 9work machine. Both running CS4.