For 2012 SCCA races, B-SPEC cars will be running in SSC (Showroom Stock C). These will pobably be among the slower cars in SSC, which says a lot for how slow they really are. I think the plan is that they'll get their own class (B-SPEC) some time after that. Not sure what NASA has planned for them. There will also be B-SPEC support races at select Grand Am and World Challenge races.
As an owner of a Mazda2, I looked into this. It's not cheap racing. Even if you build your own car - and I think most B-SPEC racers will - it'll be hard to build one for less than $16k, and that assumes that the donor vehicle was bought used and cheap (e.g., salvage 2010 Honda Fit). Want to start with a new donor Mazda2? That's $14K + tax + all the stuff to turn it into a race car (roll cage, safety gear, suspension, wheels, tires, etc.). Maybe $20K or so.
That being said, I think it's a cool class and hope it takes off. I just hope it doesn't turn into the typical SSB/SSC silliness where $6000 pro-built "stock" motors become a requirement for having a competitive car.
I know that the SCCA and others recently spent a lot of time performance-balancing (with weight & restrictor plates) the cars based on track testing so I don't see how one could say that the Mini Cooper will dominate the class. Don't forget, they're all getting competition suspensions so the very good stock Mini suspension becomes irrelevant. Plus, the Mini has a min race weight of 2600 lbs and with a restrictor plate. In comparison, the Mazda2 has a min race weight of 2300 lbs with no restrictor plate.