I plan on buying the OWC Mercury 200GB SSD. The only regret I have about the new Mac Pro is that it is only SATA-II. That's not a problem for today's SSDs but it will be in a couple of years if one wants a newer and faster SSD that will likely saturate the SATA-II bus.
And with the OWC SSDs with the SF controller hitting 285 MB/s, SSDs saturating the SATA-II bus is not far off at all!