Off the top of my head as far as current/relevant, American rock banRAB doing progressive-like things:
Comets on Fire
Battles
Bardo Pond
Animal Collective
Menomena
Grails
TV On the Radio
I would also like to say Built to Spill, Dinosaur Jr and Sonic Youth although I understand they are no longer at the peaks of their careers and might not qualify the same way.
Anyway, I just don't think the Mars Volta can be considered such a great band when At the Drive-In was far and away so much better. And although I like MGMT well enough, they haven't done anything unique in their debut and could very easily fail to make any lasting impression. Look at Bloc Party and how their doing the UK indie rock thing well for one CD made them look promising, but then after their 2nd album, it was evident they were nothing special.
You could say Battles is pretty untested too, but the band members do have a history and Battles debut was definitely far and away more unique and yet to be replicated with as much success.