According to this report in BetaNews, Sprint is planning to implement data usage caps on its mobile broadband service. Sprint will put a 5 GB per month overall use cap and 300 MB per month for off-network roaming.
Sprint states that "the use of voice and data roaming by a small minority of customers is generating a disproportionately large level of operating expense for the company. We are enforcing the existing terms and conditions for phone plans. We are placing a limit of 300 MB per month on the amount of data use allowed while roaming off network as well as a 5 GB per month limitation on total wireless data usage for Sprint's connection and phone as modem plans."
Now I don't remember seeing those usage limits in the existing terms and conditions, do you?