Motherboard makers will not officaly support them as they are server processors. However I have a 146 opteron running on a abit an8 ultra and its fine. Most people seem to run there opterons on the DFI boards if you look at opteron overclocking threads.
There is even an unoffical thread on the DFI forums from DFI saying we do not support it but it will work.
The opteron is just a san Diego core so should work with all 939 boards.
Just make sure you get the 939 opteron, and for the getting an opteron good choice my 146 is running at 2.95ghz what a bargin chip.