maybe they plan on announcing something bigger for the mac pro but it wont be released til mid 08.
the iphone was announced and shown at macworld but wasnt realeased until july. i dont know bc of att or apple reasons.
There were many factors why the iPhone was announced very early, one large one being otherwise it would have appeared on the FCC website and become "outed" which is not what Apple would want.
It's crazy to assume they're ever do that with a Mac Pro (or any Mac really), as it could seriously halt sales of the current models.