Getting back to the original post ... if May 18 somehow is the ideal date on which to announce the new Mac Pro ...
As I and others have opined, the marketing/PR strategies for the iProducts have no plausible connection with the MP timing. Completely different markets, completely different levels of interest to the general public and press (tons vs. essentially none).
My bet is that the only driving factors are parts availability and engineering resources. And it's the latter that may be suffering due to the iProducts. If you were a hot Apple engineer, which team would you want to be on? The shiny new iPad? Or the staid old MP whose outside appearance hasn't changed in 7 years? Which one advances your career, attracts attention at cocktail parties, gets you girls (or boys, whatever your preference may be)?
And given things like the audio problems with the 2009 release, whatever engineering talent is still left to work on the MP is probably (I would hope) being extra-rigorous about quality control, so that what we get in 2010 is ready for prime time. And they'll come out when they're ready, regardless of the timing of any iProduct or scheduled event.
So it may be just an under-resourced team dealing with quality-control and/or parts availability issues in what will prove to be a plain vanilla update. Of course there's always a chance of some whizbang new set of features that will blow us all away. I'm betting on the former, not holding my breath on the latter.