I actively test down to iOS 2.2 and know that my newly released updates continue to work in the field on 2.1 (due to reports from successful installations by users). (I attempt to maintain compatibility with 2.0, but believe I made a mistake with Cydia 1.1.1: Cydia 1.1.2 should fix this.)

That said, I do intend to start requiring iOS 3.1 going forward, due to the drastically decreased user community on 2.x and a killer feature that Apple added in iOS 3.1 (one of the few features added that would ever make me consider this): objc_setAssociatedObject().