I finally bought a Galaxy S4 on contract and pay for data now. Theoretically any smartphone MUST have data (if you ask AT&T) but....
I used a Nexus One on AT&T for 3.5 years up until a couple of months ago on a dumb phone plan. I have wifi at work, blanketing my entire university campus, my home, my parents house, my in laws, the restaurants I go to and even church. So I really didn't want to spend that extra $30/month for data as a poor college student when I really don't get much out of it. In other words, over 3.5 years I saved $1260 not buying data! That basically covered my semi annual Macbook Pro upgrade.
It's a great way to go if you can manage it. I still was able to make phone calls, had unlimited texting, voice mail, etc. etc. I could still use the internet just like any phone. Still download and update my apps, play games online, whatever - as long as I had wifi, which was always unless I was driving.
The way I did it was I put in a fake APN. I am pretty sure this is the only way to never get caught. Theoretically they can find out which phone you are using via the IMEI, but for some reason they never checked it when I never used an ounce of data (not even the occasional MMS text because with a fake APN, it's not allowed). I even forced my phone to only use 2G bands for phone calls just to be sure.
I'm not sure if there's a way to do it with iPhones. I jailbroke my wife's iPhone 3GS and turned off mobile data and we put in her sim card from her dumb phone in it. However, she still used MMS - and I think that's what got her caught because after about 6-12 months (don't remember, it's been a while) they told us they were forcing her to get a data plan. That was a really a sad day for me as she doesn't use data at ALL. She just uses it for texting and hardly even plays games on it. It's literally money flushed down the toilet (ok not actually literally), but oh well, that's the fascist oligopoly of carriers we have here in the US until the government tells them they can't charge people for stuff just because of the device they own (it's like Dish/DirectTV forcing you to get the expensive HD package because you own an HD TV).
If you can find a way to put in a fake APN in your iPhone, you might have a shot. But they seem to really be on top of iPhones IMEIs. Theoretically there is a central database of IMEI numbers that should've included my Nexus One, but all I know is I never got caught and I think it's because I didn't use any 3G ever, whatsoever from before I even put in my SIM card my phone was locked out of the 3G network.
I'm not 100% certain, but my Nexus One wasn't purchased from AT&T, even though it was the AT&T 3G version, I got it directly from Google unlocked, uncontracted. That might be why they weren't able to figure it out. I've never heard of anyone else able to successfully circumvent the forced data plan as well as me, but most people don't try since they want to buy subsidized phones anyway.