Folks, if you used an old iPhone backup, you're seeing the data usage from the last time you reset your old iPhone or the total usage ever if you never reset.
I reset my usage (it was at 500MB approximately after I installed from my old iPhone backup) and I've used 90MB since Thursday. This is about the same as my monthly usage with my 2G iPhone, so I'm on track for 600+MB/month, and that's despite being on WiFi 90% of the time.
I reupped my unlimited plan, since I don't know just how much I'll use with this one.