A few of us have already done it. His Mac Pro is a 5,1, which mean it already has the right firmware running on it. Further, it has the correct heat sinks that don't require any modification to fit Intel X5690 chips. It's a reasonably simple process of pulling the CPU daughter card out, popping the heat sinks off, swapping the CPUs, and reversing the process.
X5690s are still ridiculously expensive though, if you get them retail. Even with the new Sandy Bridge Xeons out, the X5690s haven't dropped in price at all (at least not from NewEgg).