Westmere-class Xeons did make an appearance in the 2010 Mac Pros, but Apple has closed the door to an upgrade path where processors are concerned in the Mac Pro. Sure, you will be able to throw in additional PCI Express cards, extra hard drives, an additional optical drive and even a RAID card, but the CPU will remain non-servicable, swapping or tampering with it will ensure your guarantee becomes null and void.
Non-supported firmware running on your machine will also ensure that AppleCare is void, but how many Mac Geniuses out there are able to tell the difference? The hack works by tricking 2009-era machines into running an EFI Firmware update that was specially intended for 2010 machines. You can also revert back to the original firmware if you so desire, but that is not a guarantee since a bunch of sub-versions of the 2009 Mac Pro EFI Firmware can be quite hard to come by these days. Anyone given it a go yet?