Rumour has it that Intel's upcoming 3.2GHz Core i7 processor will easily overclock to 4GHz on air, and even 4.5GHz on air shouldn't be a problem if you're a really good overclocker.
Furthermore, if you want to go over 5GHz and even 5.2 to 5.3GHz you need to play with LN2 but apparently this is possible and it has been done with demo CPUs. These are some of the numbers you should be reaching once the CPU is out.
Naturally, the Core i7 Extreme edition at 3.2GHz and overvoltage protection removed should be able to reach the highest clocks. It is safe to say that whatever speeds you’we seen on Core 2 Quad 45nm Yorkfield parts, you should be getting with Nehalems or you should at least be within that range.