They don't really care about various events to release products like they used too, according to a front page article some months back.
It seems to me they're simply concerned that a simultaneous product release would negatively affect the initial sales volume by muddling the focus. Single product releases have no interference that can reduce sales, and afterall, they want $$$ ASAP.
As far as being tardy with the MP, it's a workstation, not a consumer model, and it's not unheard of for the systems to ship after the official release date of the CPU models that go in them (by a couple of months, so a May release isn't going to be that critical).
But it doesn't hurt to get them out quicker than the competition to garner a few additional sales and some marketing prestige as well.
So if Apple suddenly announced the next MP now with product ready to ship, they'd not be affected as you might think (not tardy, nor undermine them).
Now if they don't release until something like September of this year, that would be a really embarassing foul-up (using existing parts, not an early release of the Sandy Bridge line).
