I expect that Apple will launch iOS5 and the new iPad together in March. That way they can give an update on the VZ iPhone performance (first month sales) and steal some of the thunder from the Android tablets releasing around the same time.
This seems spot on at least in terms of an announcement. The typical format of their keynote is exactly as you have explained - talk about how big the previous release (VZ phone) and then move to the growth and dominance of the previous gen product.
I agree that its a marketing dream to constantly stay on top of the news and not having to push as much in advertising due to the coverage it naturally gets with new announcement.
iOS announce, iphone verizon release, Ipad announce, ipad release, iphone att announce, iphone release, iOS and OS release, throw in a couple new MBP, Apple TV and mac mini or whatever to cover the time between the next year. Space these all by 2 months and you have free marketing via AP news and the need to advertise is reduced greatly