iPad 3 will be dependent on the availability of sufficient quantities of quad-core A6 processors and probably Retina displays. The way Apple now rolls out products staggered over months means that iPad 2 sales will continue to be healthy up to whatever date Apple is ready to release iPad 3.
This is another one of those techie vs "normal" consumer deals. The average consumer doesn't follow release schedules. Just like they are buying iPhone 4s today, they'll buy iPad 2s next March, April, May, June, whatever month until iPad 3 comes out. Even then they'll buy iPad 2s on closeout.
Apple sets the schedule based on their priorities. Verizon, Sprint, China Mobile, etc. have to follow that schedule. Do you really think Verizon would have refused the 5th gen iPhone if Apple had it ready in June? What were they going to say?
Yeah, right. Absolutely.
"Screw you, Apple. We just gave our customers the iPhone 4 a few months ago and now you want us to sell them the next iPhone? We refuse to do that to our customers! We reject your new iPhone. Let AT&T sell it on their own again."