It depends very much on how tessellation is used. So far it's use has been very limited e.g. dirt 2 it effects the water and the crowd. tbh it's use is so limited it's only really noticeable in static screen shots. As the tessellation was basically added by ati it's not surprising they've tweaked it to make it run well with their cards. However if more stuff was tessellated the impact would be much larger.
Fermi is meant to be a tesselation monster, much better at it then the 5870. Combine this with the fact TWIMTBP is much bigger then ati's GITG expect to see most games with tesselation tweaked for fermi in the future. This probably means a lot more tessellation at max settings, more then the radeons can handle.
That said as everything is currently a DX9c console port, and game dev's really only bother adding anything extra when nvidia/ati push them to do it. Until the next gen consoles arrive DX10 and 11 are basically an afterthought.