You can add txt messages to an iPhone plan. CHeck out this link:
http://www.apple.com/iphone/easysetup/rateplans.html
Then click on the "existing customers" tab, and you can see that you can go to 1,500 txts for an extra $10, and unlimited txts for an extra $20.
You might even be able to go to the 400 txt plan for an extra $5 like a regular phone, but I am not sure about that...
I have the basic phone plan (450min/5000 nights and weekends/unlimited M2M/rollover). I got the data plan with 1500 texts cause 200 isn't enough for me. I end up paying 82 a month plus taxes and whatnot.