It seems like the SATA3 ports work at full speed under MacOS 10.6.4. I just got a Crucial C300 SSD which is capable of SATA3 and it reads at speeds of 340MB/s. On windows 7 it reads with 380MB/s. I think the 40MB difference is the lack of a real SATA3 driver in MacOS. In System Profiler the SSD shows up under "Unknown AHCI Standard Controller", AHCI Version 1.20 Supported.
Phenomenal speed report but too bad speed increase is only with Read. Hopefully when more companies release SATA III interface on SSD drives, they will increase Write speed as well.
Personally, I decided to go with Kingston SSDNow V Series (SNV425-S2/128GB 128GB SATA II Internal Solid State Drive) - On sale this weekend newegg for 219.00 - Then when new drives are out with more competitive prices and better speeds I can upgrade to SATA III and put the Kingston in my MacBookPro which doesn't support SATA III.
I think that the lower write speeds on SSDs are okay since most conventional harddrives do not read faster than maybe 110MB/s. So if you copy from a regular HDD to a SSD there will be no loss of speed.