SSE4.2 actually sounds like a decent way to do it also; detecting the presence of SSE4.2 will surely detect the presence of a Nehalem core (and, likely, all later CPUs).  I'll try using that instead.

Then again, I can't commit the code until Nehalem release anyways, since the Nehalem-specific optimizations I'm working on probably contain enough information to violate the NDA if I released it earlier.