I’m close to the end of my first AT&T billing cycle to consist primarily of iPhone 4 use. So far I have used about 1.5GB of 3G data with wifi disabled, and at this rate I should come in at less than 2GB by the end of the billing cycle.
With my iPhone 3G I usually used quite a bit less since it had slower hardware and a slower Internet connection.
I think I use my iPhone more heavily than most people, so the 2GB plan should be plenty in most cases. But if you’re using a lot of Pandora, YouTube, etc. on a 2GB plan, you might want to keep wifi on to avoid overage charges.
And the 200MB option? If you do anything more than basic email and Twitter you’ll almost certainly want to keep wifi on.
Ultimately 2GB would be enough for me but I’m not letting go of my unlimited plan just yet. To me it’s worth paying the extra $5 to not have to worry about it.

what’d you mean by “Ultimately 2GB would be enough for me but I’m not letting go of my unlimited plan just yet.” ? because i’m plannin to go back to an iphone and get the iphone 4 and get the 2g… but i really want the unlimited data plan. does that mean i can get the unlimited plan?
You can’t get the unlimited plan as a “new” customer but if you already have it at the moment you can keep it.