Let me refine that. They can be different models, but they can't be ANY model. So far the line-up goes from the X800 Pro to the X850 XT PE and they have to be PCI-E.
also supposedly its suppose to be much more stable then SLI. I've heard horror stories about SLI, and some really great stories about it, supposedly Crossfire won't have nearly as many flaws, but nothings known for sure yet.
(I've heard of kids both vid cards burning out because of SLI, and a lot of other stories)