Is it possible to use an iPhone without data?

I recently started at a new job and now work at home four days a week (I only go in to the office once a week for a company meeting). Because I'm no longer commuting regularly and obviously have wifi in my house, I'm using almost no data on my iPhone 4 anymore. I'm already on the cheaper $15/month data plan with AT&T, but I don't even max that out anymore; as of today, which is halfway through my billing period, I've only used 26 MB out of 200.

So what I'm wondering is, is it possible to just get a Tracfone SIM or something and use that for voice, then disable all 3G/data services on my iPhone? I already use almost no voice minutes as it is (I got the iPhone primarily to use data), and I'm now on wifi almost 100% of the time, so I'd lose basically nothing compared to what I have now - it would just be a fraction of the cost.

I'm assuming there's some catch here that prevents me from doing this, otherwise probably lots of people would be doing the same, but I figured I'd ask!

This is most certainly possible. The first few months I had an iPhone, I didn’t have a data plan either.

In Belgium (where I live), iPhones are generally sold separately, without a data plan / SIM — they’re officially unlocked iPhones.

So, the answer to your question is: yes, it’s possible to use an iPhone without a data plan — but you can only change the SIM card if you own a SIM-unlocked iPhone.