Do you have an early 2009 Mac Pro and want it to run Westmere Xeon CPUs? According to Ars Technica, an EFI firmware hack lets you do just that, among many other niceties.
The firmware enables three main features for 2009 Mac Pro users. Perhaps of primary interest is that users will be able to install 32nm Westmere Xeons, including six-core variants used in the high-end 2010 Mac Pro, into their older machine. Single-socket machines can use W-series CPUs, while dual-socket machines will need dual-QPI enabled chips including the E5600 and X5600-series chips. (A source who applied the firmware update told Ars that Westmere CPUs are identified with "B1" stepping in the identification code.)
Even if you didn't plan to upgrade the CPU, though, there are other benefits. Some Nehalem CPUs can support 1333MHz DRAM speeds, but are limited to 1066MHz on the older Mac Pros. The firmware update will enable the full 1333MHz speed if you have a CPU and RAM combo that can support it. And, the updated firmware will channel audio signals through the Mini DisplayPorts on installed GPUs, making it easier to use with HDMI displays.
Take note though, swapping the CPU or flashing an unsupported firmware would mean voiding your warranty.