If those case designs are accurate, I'm not sure that this is a major redesign of either notebook line. It seems more like a logical progression, like the transition from the iBook to the MacBook. Apple already highlighted their move to aluminum in their consumer lineup with the introduction of the aluminum iMac. If there are any design cues taken from the Air, then--again--it's already been showcased.
Of course I'm just basing these on the previously leaked images, which themselves don't tell you much. I'm just trying to say that even if the look of the new notebooks was significantly different, it may not warrant an event because Apple has already more or less announced the direction of it's industrial design. We're just waiting for the rest of the products to play catch-up.
I know the iPods got an event for their redesign, but then the iPod touch had some fairly significant new features, and I think Apple wanted to hype up the possibility of the touch as a gaming device.
In addition to that , Apple mostly doesn't do media events just to update hardware there is always some kind of software release or update too (at least in the last couple of years). However if teh macbook(pro) gets a redesign(and it will) it will be showcased because it has been a looong time since it drasticly changed looks.....hmmm it's a tricky one , even for Apple I guess.