Technically, car seats are supposed to have an ECE sticker specifying version 44.03 or .04 though I think most US approved seats will pass the EU norms as well.
From what I understand, the TCS doesn't feel it's necessary to approve a vast number of seats and doesn't think that keeping your kids rear-facing for any length of time is important. Apparently, for this reason, as long as you have a seat which has foreign approval, it should be allowed here, too.