AT&T Wireless has pretty explicitly dropped the hammer on Slingbox owners, amongst others, via their revised terms of service:
customer initiated redirection of television […] to a mobile device […] is prohibited
Some speculate this language is in response to oversized and unintentional roaming fees. Others believe AT&T has “oversold” or is approaching their network capacity and this is a way free up bandwidth. I have a more pessimistic, Net non-Neutrality theory in that AT&T would prefer we subscribe to their limited selection of MobiTV television content in lieu of streaming home video (which I’ve already paid for). Regardless of the reason(s), I’m not a happy AT&T customer today.