I do quite a bit of freelance photography so I like to show the client my website which has a lot of photos so it eats up a lot of data. And off the record I like tethering for those times I'm on the road now, after I updated to 3.0
Since I can't stream music on my work PC, I've become a hardcore Pandora and Shoutcast app user over 3g. My monthly use has gonne from about 400 MB to about 1.5 GB
I usually try and max out my data cap on my plan. Went from 500MB to 1.1GB limit with 3gs upgrade. To get to 1.1GB will probably require tethering.
Australian carriers, all have data caps...
But also since I have gotten my uverse with a new router I use my wifi quite a bit more than I used to. My wifi didn't even cover my small house all the way before and I would use edge in my dining room.
I use between $19,000-23,000USD worth of data per month, according to my carrier. Of course, that's adjusted down to about $50 (flat-rate data) ... In terms of MB, I think it's only about 300MB/month. Not that much, really.