When the new Mac Pro came out and was found to have a user-upgradable CPU, Mac supply distributor Other World Computing published a small article showing the system will readily accept a new CPU and be able to function just fine. However, this was only one of a number of different potential upgrades that might work with this system.
Since its initial publishing, OWC has been at work installing and benchmarking a variety of different CPUs in the new Mac Pro. The distributor is in the process of compiling a table of compatible CPUs, and lists four options as either "OWC Tested" or "OWC Qualified" for use in the new systems. There are still a number of additional CPUs to go, and it will be interesting to see what OWC finds.
Here are also OWC's benchmarks from the latest upgrade tests.
It is worth noting, that while these are CPU upgrades that are working in the Mac Pro, the CPU is not considered a user-upgradable option, and therefore changing the system in this manner will void your warranty. For those who enjoy tinkering and have a backup plan, this may not be a problem, but if not then you might wait for the warranty to expire before considering such an upgrade.