http://buyersguide.macrumors.com/

Brilliant site. It basically goes through the different Apple product lines and keeps track of how long it has been since the line was last updated.

Apple, like most companies, have an internal update release schedule to keep the market engaged with their product. They don’t let the public know what the exact schedule is, but the guys who run this site keep track of it. If the product you are thinking of buying is at the end of it’s cycle and is probably about to get superseded, a) it is about to become a whole lot cheaper, and b) if you’re a cutting edge type of buyer you’re about to get sorely disappointed.

Worth checking out…!