I have used just over 50Mb in my first two weeks of iPhone ownership (according to the Usage page in Settings), so I guess I'm using between 100 and 120Mb a month.
I use solely wi-fi at home, then a mix of GPRS and wi-fi at work, and GPRS, EDGE and wi-fi when out and about.
My old Orange contract charged £3.00 per Mb, so I would already have spent £150 on data 😱 Suddenly, £35 a month doesn't seem too bad 🙂