My take on the 3G iPhone goes something like this - if Apple is planning a revamped "iPhone MKII" then the anniversary mark is as good a time as any to plan a new release. With that in mind, Apple would be placing orders with manufacturers right about now. Who knows, maybe Apple is anticipating a cooling off of interest in the current iPhone and planning accordingly.
On the other hand, the MK I iPhone is still young. Sure, there are a lot of features that Apple could add and tweak, but these could wait another year or so without putting much of a dent in the sales. Also, by the 24 month mark users would were early adopters of the first generation iPhone would be getting ready (both emotionally and fiscally) for an upgrade.