I think the every-other-year plan is quite sensible. Each update is "good" so every 2 years gives you a "great" update.
And hey, if that lines up with your 2 year AT&T contract expiring, even better. Just a side benefit there, really.
I have a 2008 that I'll be switching to a 2010 when it comes out. (Also, let's just use years for the phone models. Much less confusion that way 🙂)