It really depends on how you use your devices. As noted so far in this thread, some people chew through massive amounts of data very quickly. I always thought I was a moderately heavy user of my phone, but I typically only use 800-900 MB (with usage going up to just over 2 GB when I go out of town for a bit). Obviously nothing compared to other users, but I spend 80% of waking hours in places with easily accessible WiFi.
With my iPad, I was much more likely to rent a movie from iTunes/rip a movie in my collection if I knew I was going be traveling for long periods of time without free downloads. That way, I saved my limited 3G data for more routine/essential surfing/email/etc. Just because you have wireless cellular data doesn't mean you HAVE to use it for everything.