Does an unlocked smartphone need a data plan?

I know that cell companies force customers to have data plans if they buy smartphones. My question is, if i buy an unlocked smartphone, like a samsung focus, and use it with my regular tmobile plan that doesnt have a data plan, will i be forced to add a data plan? I dont need data everywhere. Just having it when i have wifi is enough for me. i just dont want to be forced to pay for a data plan