I regularly use over 3gigs a month. I don't tether, but I have a one hour commute to and from work. That usage is mostly Pandora and some Podcasts.
I listen to podcasts 10 hours a day (mostly talk radio), but my data usage is typically under 1 gig a month. I have BeyondPod download all my podcasts at 2:00 AM on my WiFi. I once tried streaming the podcasts using Stitcher on my Droid, but rejected that after a couple of days because it really burned through my battery. If I downloaded all my podcasts on 3G, I would easily be using 3-5 gigabytes a month, but with doing it at night on WiFi, my 3G usage is low.
Instead of Pandora, you could try Slacker. I've read that their premium service allows station caching, which you could do on your WiFi for playback when you are on the road.
