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.
You will then be shown a nice bar graph of your data usage by the MB. Unfortunately, data isn't available before November 2009, but this is still very helpful information when deciding on a data plan.
You can also check your usage with AT&T's myWireless iPhone application [free - iTunes link], but it will only show data usage in your current cycle. This will be especially helpful after you've chosen a plan and you want to keep track of how much of your data you've used.
It looks like I'm generally under 200 MB and I'm highly considering the $15 plan. How much data do you use? Do you want to tether? Will you switch?