As far as I know, at the moment, Sprint cannot accurately determine on there end what's happening to the data on the phone end (i.e. is it being digested by the phone or passed through to a tethered device). They can probably guess and make some assumptions based on processing power of the phone, connectivity, volume of data, etc, but I don't *think* they can accurately determine what happens to the data they funnel to your phone.
Now, that being said, if any of you would open up the contract copy they included in the box with the NS4g, you will see there that they specifically limit the data usage to the phone and only the phone. I don't have mine in front of me to quote word for word, but to paraphrase it as I recall, it said something to the effect of 'you can use the data for you phone and only your phone, not other device is authorized by us, doing so violates your contract, and we reserve the right to terminate the contract if we think you've done it.'
So can you? Yes. Does Sprint 'allow' it? Not without paying for the service. Is it violating your contract to tether without paying for it?" Yes. Can they catch you? Dunno...