For 2011, sata 3 is supported.
Edited by un1b4ll - 6/13/11 at 11:49pm
You have to use a product called an Optibay to have a 2nd hard drive, it replaces your optical drive though. However, it does come with an external enclosure so you'll still have access to the SuperDrive when needed.
The existing hard drive slot is the only slot that is bootable in the system outside of the USB/Thunderbolt bus, so make sure you install your boot drive in the original location, or if you just need extra storage put it in the optibay.
Hope this helps, if you have any other questions I can answer them.
Seagate Momentus and WD 2.5" Black drives are probably your best choice, 750Gb gives good platter density at a good price.
If you're looking for best performance over pure size, the Seagate Momentus XT is the king of the hill for 2.5" mechanical drives. It's partially solid state for common random reads, which really speeds things up. Also, it's entirely handled by a chip on the drive itself so the fact that Snow Leopard doesn't support TRIM doesn't hurt you at all with this drive. It would definitely be my choice of drive, if that bears any weight.
Don't worry about not getting a sata 3 drive if you're not going to get a current-generation solid state, you won't come near the bandwidth bottleneck of sata 2 with any mechanical drive, and most solid states.
Edit again: Here's the Momentus XT in action