I'm using T-mobile, had the sidekick previously and called the CSR once I had my iPhone they switched my internet to the 34.99 Smartphone plan which is unlimited internet, e-mail, and text messaging.
My voice is the myfaves. If you wanted to remove the Smartphone plan then you could only use the iPhone on a wifi signal. That would work at home/work/school but if you don't have a wifi signal then you would only have voice.
I don't think you can remove the voice and just keep data? If you have a google voice number and myfaves then you can add your google voice number as a fave and research that online... there are guides on how to set it up properly.