Environmental conditions have a lot to do with it - extreme temperatures tend to shorten a battery's lifespan - heat seems to be harder on batteries than cold. Also, a battery that is left to go completely flat regularly or is overcharged often will not last as long. My experience is that a battery should easily last 4-5 years with proper maintenance. But then again, there is always the faulty one that dies early...

I bought my '05 last year and the dealer had just put a new battery in it because the original died. My car had never been sold to a private party - was either sitting in parking lots or driven occasionally by dealers.