How long does it take to make a context switch?

That's a interesting question I'm willing to spend some of my time on. Someone at StumbleUpon emitted the hypothesis that with all the improvements in the Nehalem architecture (marketed as Intel i7), context switching would be much faster. How would you devise a test to empirically find an answer to this question? How expensive are context switches anyway? (tl;dr answer: very expensive) The lineup