Many of us use their phones at work as work firewalls will block certain sites (such as facebook or twitter). I do some light tethering while at the ladie's place as she has no broadband (gasp!) and no open networks within range. Most of my traffic is endless surfing, tons of streaming audio (Pandora is on quite a bit) and I have wifi disabled on my phone, which really saves on the battery life.
I push around 4 GB a month.
Oh yes, I recently got a new headunit (car cd player) that will play & control mp3 players hooked via audio cable. I was able to get my droid to output my Pandroa to my car speakers!