My first full month I used nearly 900Mb and that wasn't really doing anything special! Think a lot of people get caught out by the wireless switching off when the phone is sat there by default so everything in the background is via data, had a few wireless 'issues' at home so often would switch to data if I was doing something, daft I know, but once I got that bill, (which cost about a tenner extra) I downloaded netcounter from the market, you can't believe how fast the hero uses data! So I switched off mobile data and will only switch on when needed! Bit daft I know with an 'unlimited' plan, but 500mb shouldn't even be considered close to unlimited for this new generation of smart phones!
ps. that wi-fi fix app has certainly made my wireless at home far more stable