I use about 3GB/mo, last two months I did 3.8GB on my unlimited plan, but that is b/c I was using YouTube HD streaming with a JB program called 3G unrestrictor and it allows the iPhone to think is on Wi-Fi letting you watch you tube videos in Hi Def.
Overall I've noticed my average is like 3GB per month, but I have unlimited data so I don't get charged an additional $10 for that extra Gig.
The people who are getting in excess of 20GB/mo are people who are definitely tethering their iPhones using either myWi or other JB tethering programs out there. I also have a myWi license, but would only use it in case of an extreme exigencies, like my home DSL going down which it did two nights ago as my motorola DSL modem gave up, burned up in smokes and had to wait two days to go to AT&T store to get another DSL modem/router from 2wire. But again if these people who are getting in excess of 20GB/mo are probably either using their iPhone's data as their home Internet or are just too negligent.