My wife and I already do nearly $100/mo with Cingular - $60/mo for 850 minutes, $10 for the 2nd line, $2 each for (50?) text messages, $20/mo unlimited data for me. My company discount takes care of all of the taxes plus a few bucks (18% discount). I've got an 8125 WM5 device, but I signed up for the data plan while the SIM was in my old phone.

So, if you're already paying a monthly cellphone bill, then the only costs you'd incur would be the incremental charges and the device cost. Without looking at the details of the family plan, I could switch to the $79.99 plan on my line, my wife keeps her current phone for $10, drop the data and text messaging, and I'd basically be at the same cost out-of-pocket. Personally, I won't be switching because I've already got a perfectly good 30GB iPod and 2GB Nano to cover my music needs.

Heck, if you've got a non-corporate Blackberry/smartphone just to get e-mail access with a $45/mo data plan + voice plan charges, the iPhone would be cheaper over a 2-year contract.