My roommate used to stream Pandora on his iPhone for 6+ hours, 5 days a week for over a year. That plus apps, and surfing and everything else he used approximately 6GB that entire year.
If the price went down to $15/month for 2GB that I think would alleviate most of the issues that people have with this. Leave the 5GB or unlimited in at $30/month, but a $5 drop for a 60% reduction in data... weak.
I don't even think that I will ever top 2GB/month, and I am happy to save $5/month... but I can understand where people are coming from. However, and this is a slight aside, but I keep seeing Zerocool all over the place, sometimes talking about how iPhone apps are for kids, and Android apps are great for real work (paraphrasing) not trying to start a war or anything and I really don't care about his opinion vis a vis the iPhone apps, and have seen him a couple of times complaining about the data issue.
Seriously dude, what the hell are you doing on your phone? And if you are doing this much, why haven't you gotten a laptop/netbook with a 3g card yet? Seriously, if you are consistently moving that much data on a 3-4" screen... something is wrong.