I normally only use between 500mb to 1gb a month, including all of my work and personal email being pushed, reading news daily and a little bit of youtube.
However last month it suddenly went to 6GB, It turns out, I was rooted, and on a custom ROM, the phone kept trying to download the official 2.1 ROM every couple of hours, thus racking up the 6GB useage in 1 month. So after fixing that problem, this month, so far 1/2 way through the billing cycle and it's already showing 2gb of use. The phone is not trying to download a rom, but I don't know where all that usage comes from. Looking at the details from myverizon. it shows most days as just 10 MB or so of use, but 1 day this billing cycle it had 1.5GB, and the following day was just under 500MB. Not sure what it was trying to download those days.
It would be helpful if there was a tool within the android phone/OS itself that shows you how much data it is using or has been using. So you can gauge.
If they go to a pricing plan with limits, many people will get surprise bills.
6 months ago I got a $400 data bill (not as big as some of the other people) because for some reason, verizon put all my data usage on my wife's line that doesn't have a data plan. while mine had an unlimited plan. Took several calls and many hours to get it straightened out.