The good news is, existing iPhone users with the $30 unlimited data plan can keep that plan even if upgrading to the next generation iPhone HD/iPhone 4G this summer -- unless you want to tether, at which point you'll need to switch to one of the new AT&T tiered pricing data plans for smartphones.
If you're trying to decide whether or not you should make the switch, the first question that comes to mind is "how much data am I using now?" You can find out very easily from AT&T's website and TiPb is here to show you how.