A year ago and prior, I would use about 150MB a month. About a year ago I got a new job that sends me out of the country for 11 days at a time, and therefore I don't use data when beyond the US borders. Oddly enough, however, the 8 or so days a month I'm home I tend to use more data than I did when I didn't leave the country. It's almost a regular occurrence to get a warning from AT&T that I used 90% of my data.
So to answer your short question the long way: I use 200 to 250MB a month.
I may be moving projects in a week, and my company is considering send me either to Colorado or the UK. If I go to the UK, I'm sure I'll have roughly the same data usage. If I go to Colorado, though, I can easily see myself going over the 2GB limit. I think I'll threaten to jump ship to Verizon (which is an honest thought of mine) unless AT&T gives me unlimited again.