Yes, that is something that should have probably been researched before the purchase, especially if that was its intended usage. As of this moment, there is zero compatibility with AMD GPUs and the AE raytrace engine. There are no hacks or modifications to make it possible. I have, however, heard second-hand that Adobe was pushing the mac pro hardware in their booth at NAB, so maybe they have something up their sleeve, but I don't remember hearing anything specifically about converting AE RT to support OpenCl. I would imagine it would be quite the endeavor and a big announcement.
  You should still see good performance from Premiere Pro with the Mercury Engine for encoding and some GPU effects, although honestly I have no idea. Like Elliot says, the thing really seems built to drive just one app for now: FCPX. I just recently spoke to another lister who purchased a new mac pro and returned it after realizing some of its limitations.
-TG