It comes down to how bad you
truly need a new machine. If you can hold off, then do so, and see what's announced to see if it's worth it vs. the current system (which you may then be able to find at a discount).
But if you're earning a living with the system, and having it or not is the difference between profit and losing business, buy now and be done with it.
BTW, in terms of a desktop only, Thunderbolt isn't really useful. PCIe slots can allow for faster throughputs and less expensive solutions than TB based peripherals (i.e. if eSATA's fine, then get an eSATA card), as the available information shows TB is more expensive than initially expected. For example, available information puts the TB chip cost at $90 to a manufacturer, which will push MSRP's quite a bit. In terms of I/O throughputs exceeding TB, you can get faster drive throughputs via a hardware RAID controller and enough drives (right configuration for the requirement), better video bandwidth with other interconnect standards, ...
Now if you mean to share high speed peripherals (
example) with both a laptop (location video capture for example) and a desktop, then having a PCIe TB card (or a desktop system with built-in TB) would have merit. Matrox has announced a card (not sure if it's actually available, but the MSRP is $299 USD -
source).
As per the newer SB Xeons that will be used, their biggest advantages are the potential for slightly faster clock speeds, and improved system I/O between the CPU and both the memory controller and PCIe lanes (take a look
here for more details). No release of any price information yet, but I wouldn't expect it to be inexpensive in terms of quantity pricing (more expensive than current offerings due to the larger pin count on the socket = more complicated CPU). Where it can work out, is if you're able to utilize all the cores fully.