Anyone who is wondering about when the Sims 3 will come out for Mac, should check Aspyr's web site frequently. Aspyr converts PC games into Macintosh compatible ciders. The requirements for the game are:
Processor: Intel Core 2 Duo @ 2 GHz
Memory: 1 GB
Video Memory: 128 MB (Dedicated Graphics Chipset) That means no white macbooks or Mac mini's.
Any Intel graphics chipset will be unsupported.
No OS requirements have been posted but I presume it will require Leopard 10.5.6