I don't know about other carriers, but T-mobile actually makes it pretty obvious. T-mobile seems a lot less consumer-hostile regarding these kinds of things than either AT&T or Verizon in my experience.

http://www.t-mobile.com/shop/plans/Cell-Phone-Plans.aspx?cat...

Though it looks like the $40/mo plan now has a 5GB cap; I got mine when it was unlimited.