I'll post at odds with Andy!
The MAC doesn't seem to really have any teeth yet. Once collected items hit the exporter, all the items, certified or not, get mixed with items from other collectors, certified or not. Once those items are exported, they hit wholesalers where they are further mixed with other items, certified or not. And once they hit the LFS, even more mixing happens. So, an LFS MAC certification doesn't really say anything about the the livestock/liverock is certified. And just because an exporter is MAC certified doesn't really mean that what you are getting at the end of the supply chain is actually from that certified collector or exporter.
I really wish I could get behind the MAC, but currently the do very much seem like a rubber stamp organization to me. Perhaps in the future they will be.

If you are interested in more MAC info, the Industry Behind the Hobby forum on reefs.org is chock full of discussion.
Having seen WS's guys collecting live rock in person, I am concerned that their promo doesn't jive with reality - and I really don't buy the idea of a collection 'circuit'. I think all the collectors in the SP say they do it, but I think the reality is very different. From what I heard from locals in Tonga, WS has had just as much negative effect on the environment as any other operation, and that he only cares for promotional value. This is of course, hearsay.
Live rock in general is getting more and more tricky with a lot of it sitting in shipping containers for weeks or months and then being sold as 'live'.
In reef keeping, there are at least a couple schools of thought on live rock. Some people love it, some are worried about bad hitchhikers, some think its almost useless because most of the stuff on it will be rotting due to poor shipping methods.
The Tampa Bay stuff has mixed reviews as well due to where its left in the ocean. The thought is that its loaded with phosphates.
Live rock and its impact on natural reefs is a constant topic of discussion for reefers. You'll get as many opinions as people.
Mucktopus and Andy covered anything else I would otherwise say.
My opinion: buy the rock that you like the look of.
