Interesting how early poll results (55 people voting) suggest that the octad 2.66 is the least popular configuration, which is surprising to me. I would have thought that one would have been the happy compromise between clock speed and multi-cores. In fact, it is the system I am thinking of buying...
If you need stupid speed and its going to make money for you, nobody is going to stop at the 2.66GHz 8 core. The extra $1400 is hardly compromise when you have the option not to spend it at all for a very similar performing 2.26GHz unless you need as much speed as possible as your livelihood depends on it.
In the threads on comparing or trying to buy the previous 2008 Mac Pro model, the 2.8GHz and 3.2GHz appear to be the only models discussed.