SaltyNZ: I'm confused by people who claim to use this much. I work for 2degrees - basically I get unlimited free data, and I'm on 3G all day. And I only use about 500MB a month max.
I use about 750mb per month on my iPhone. Of that about 200mb is general mobile apps and browsing and the remainder is streaming radio that I listen to during my commute to and from work during the week. Remember, the iPhone lacks a conventional FM receiver so streaming is the only way to listen to mainstream radio.
On rare occasions when I'm out and about with my Macbook Pro I may tether it to my iPhone which obviously means using a bit more data. I also go through about 500mb a month on my iPad but obviously that's a separate connection.