I make stuff last a long time... I'm taking the same attitude as I have done with my MacBook. I bought it in mid-2009, and I'm definitely going to make it last until 2014 at least (until I've finished university), unless something catastrophic happens and it breaks beyond repair...
I am not trying to talk you down, but a laptop lasting 5 years is a lot easier than a smartphone. You are certainly going to lose support around year three, at the latest. This means no new versions of iOS, apps will slowly become outdated, etc. If you are ok with this, go for it. I just thought it might be worth being informed. If you are certain you will go five years with this phone, I too would recommend waiting for the five.