Almost every well known brand out there, Trek included, makes a good carbon frame.
The question is obvious, how good of a frame are you looking for? Which leads into the question of how much money you want to part with.
There's certianly not much wrong with a Cannondale Six, a good value carbon frame.
Until you've ridden the SuperSix.
Geometry is another consideration, do you want something more relaxed for longer rides, check out the Giant Defy or Specialized Roubaix.
Or something a little more agressive, Giant TCR, Speicalized Tarmac, or the Supersix.
You can roll the dice and pick something up online, or go to a dealer and test ride a couple of frames until you find the one you like.
This being your initiation into the world of carbon, I'd go with the latter.