Business 2.0 has a nice guess on how much the iPhone's data and voice plan is going to cost. Instead of an all-you-can-eat plan that Wilson speculated on, Biz is estimating based on current BlackBerry fees.
The status now: between $34.99 and $79.99 for data and $39.99 to $99.99 for voice. If you're going for the lowest numbers, that's $80 a month sans fees and tax. Not too bad, but all that gets you is 450 minutes a month (your data is unlimited). For non-BlackBerry devices, unlimited data can be had for $19.99. For devices like the Treo who want BlackBerry connect, it's $49.99.
Here's some good news. It seems AT&T's cancellation fee will only be $175, which is much less than what you'd pay in unused data and voice in your second year of service. It may be better just to cancel and eat the $175.
In the end, if you last the entire two years, you're going to wind up at least $2,218.12 (not including tax, plus $100 more for the 8GB model) poorer if you're getting the iPhone.
Apple's iPhone: Adding Up the Costs [Business 2.0]