I wouldn't assume it's actually unlimited. One of my users was told to leave by Sprint for using too much data on his iPhone. At least they waived the ETF.
Well, I have myself using plenty of data on a daily basis and my daughter streaming stuff to her iPhone (wife doesn't do much data).
It clearly said on the forms that I signed that I had unlimited data BUT that data was restricted to handsets. If I wanted to activate TETHERING - THEN I would be subject to a surcharge if I went over a limit (which I believe was 5GB but I'm not entirely certain).