I was about to say I just read it was only with DX11 hardware. Well, what's the point? You still have to buy a DX11 card and you might as well use MS's stuff, so you get no issues, right?
Well, the HD 5000 series are the first AMD GPUs with official tessellation support, but a tessellator has been built into AMD GPUs since the R600 (in fact, it even went into the x360 Xenos GPU). From what I've read (it obviously will never be verified) is that the original DX10.0 spec was to include tessellation (hence why AMD included it). However, nVidia didn't add such a component into its "DirectX 10.0 certified" g80 architecture, pressuring Microsoft to lax the API and shove off tessellation (and other features) for DirectX 10.1 and 11.
Using the Linux Unigine OpenGl demo (not sure with the Windows OpenGl renderer), one can use the tessellation option with a pre-RV870 GPU (not an option with DirectX 11). Granted, the framerate takes a nose dive, but it is possible. It is unclear if the Unigine demo is using the built in tessellator of a pre-RV870 GPU (RV770 in my case) or just doing it in software. Because the Unigine demo (and Ubuntu Karmic and newer) require an R600 (or more recent GPU), one cannot see if tessellation would even work with a R500 or older GPU. It'd be cool if someone with a pre-Fermi nVidia GPU tries to run the Unigine Linux demo with tessellation enabled and reports their results.
Finally, one more thing to keep in mind is that many stubborn people (
) chose to remain running Windows XP, and thus they loose out on the DirectX 10/11 features (regardless if their card is capable of them). Should a game support OpenGl 4.0, it will run in XP with graphics comparable to the same game being rendered on Vista/7 under DirectX 11.
As an aside, I would like to do something I don't typically do, and that would be commend nVidia on their Fermi tessellator. From benchmarks I've seen, Fermi's first generation tessellator eats the R870's tessellator for breakfast, yet AMD has had tessellation experience since the Xenos GPU (as I mentioned before).