One of my co-workers has T-mo and the unlimited plan that he signed up for is nice and all, but after (I think) 5 gigs of data, he gets throttled down to a 2g connection. He doesn't use his phone that much so I don't think he will have to worry about it.
If Sprint ever decides to make that decision to do a data cap, or a throttle, hopefully they will do like AT&T. IA couple of my friends are grandfathered in to an unlimited plan. No throttle, no caps.