Here in the UK, mobile operators include different amounts of data with the iPhone data tarrifs – some even offer unlimited (with fair use making them not unlimited at all). Check the terms very closely, to know exactly how much data is included with your tariff on a monthly basis.
I bought an iPhone 3GS just under a year ago (20th June 2009) and my data usage in that time has been just 908MB, sent and received. That’s not much. I do use WiFi when at home, though.
I have an Exchange push account (with quite a bit of mail flow), random other push apps, I use the browser quite a bit when out and about, I use the maps, and I send a fair few e-mails, often including pictures etc. I’d say my use is fairly typical.
My average data use (none-WiFi data) a month is 75MB. So, even if you doubled the use, it’s still only 150MB. You could say you use data on your iPhone five times as much as me, and it’s still only 375MB a month!
The allowances are often quite high (for a phone) – I’d be surprised if you hit the limit (at time of this post) offered by the mobile companies, certainly in the UK, unless you’re tethering it and using it as a modem, or constantly streaming video such as YouTube etc.
What’s your data usage? Let me know in comments below!