If you’re planning to upgrade your iPhone, iPhone 3G, or iPhone 3GS to the iPhone 4, and want to take advantage of AT&T’s new, cheaper limited data plans, here’s a tip: don’t use your iPhone 4 for a day or three before checking the “Usage” meter that shows you how much cellular data you’ve consumed. (Settings > Usage > Cellular Network Data)
A good pointer as of now: start your iPhone 4 fresh rather than transferring over all of the past settings from your earlier device—it looks like there are some benefits to doing so. But if you want to ignore that advice and preserve your old settings, make sure you reset the usage meter on the aforementioned Settings > Usage page with the Reset Statistics button. Otherwise, you’ll carry over the call time and data usage from your old phone, and get the shock of your life when you see that your brand new iPhone 4 has supposedly consumed enough data already to destroy your monthly phone plan.