In the words of Cédric:
A small message to let you know that I have just received my new MacPro Nahelem Quad 2,66 GHz. I have just tested a set of 3 memory card 2GB OCZ DDR3 PC10600 (FSB of 1333 MHz) NON-ECC. The machine accepted them without a problem. They are recognized as 1066 MHz, the absence of ECC does not create any problem.
Being surprised that this memory functions, we asked him to make a hardware test on his machine. It ran in a normal way without posting any errors. Even though this makes it possible to find less expensive memory, we strongly advise you to use ECC memory that avoids the small risks of hanging related to a corruption of the data on a card.