ThinkSecret isn't nearly as reliable as they used to be, but they are speculating that the Mac Pro will use Conroe rather than Woodcrest, which means it can't come out before Conroe does. Conroe is expected in either July or August. Woodcrest is coming out sooner though, so they could be less than a month away if they use Woodcrest.
I agree with Zap2, I want the high end Pro Mac to maintain quad cores, and I don't think Conroe is capable of that.