First post and did a search and couldn't find anything definitive.

Looking at getting a 3G Ipad2. I want the 3G simply for when I travel which is not too often. My question is, with either AT&T or Verizon is the pre-paid data plans truly pay as you go? For instance, if I go 3 months without a need for 3G will I be billed? Or do I have to cancel/reactivate each time with associated activation fees?