And why would you get an iPhone or smartphone without a data plan? Of course you want a data plan with it.
I own an (unlocked) iPhone on T-Mobile and have no data plan. Of course, all I really wanted or needed was a phone with an iPod in it. I spend maybe $15 a month on prepaid minutes.
The only thing I feel like I'm missing out on is being able to stream internet radio stations in my car, and while that might be awesome to do I don't think it's worth an extra $60 a month. If I ever need web access, it's not like there isn't a Starbucks with WiFi in my vicinity anyway.
Plus T-Mobile's voice+data+text plan is $65/month, so if I got that there would be almost no point in having an unlocked phone anyway. Might as well pay the extra $10/month to AT&T and not have to deal with firmware upgrade issues. Though I hear that AT&T's customer service is the worst in the business, so maybe you'd be better off running an unlocked phone on T-Mobile.