Where's the BEEF!!
Here's why:
1) Apple won't release the MP before May 18. It needs the iPad to establish a solid press foundation and won't risk competing against itself. The iPad needs these two weeks to transform from "that's neat" to "yeah, ok I need that."
2) Apple won't release the MP after later May. This would take too much energy away from pre-WWDC buzz (and it seems reasonable that the buzz will be iphone oriented, as it should be).
3) If Apple releases the MP shortly after WWDC then they compete with themselves again: bad idea.
4) Apple should release the MP before July. They have NEVER taken longer to update a product line (see the buyer's guide on this site--assuming the buyer's guide is accurate--is it, btw?).
My money says they'll release a new MP on May 18 in mostly the same case (expect minor mods), and with processor/memmory/hd bumps that you'd expect (and a marketing campaign explaining in detail why this is the fastest Mac ever blah blah blah with lots of neat-o processor images and mapping illustrations).
Anybody want to place the odds?