There isn't, but one would note that O2's terms and conditions for mobile internet state that you're not allowed to use it for streaming media. I'm not aware of anyone having had O2 say anything, but they would be quite within their rights to block such a service or charge you for it (as I understand it this is one of the reasons the BBC iPlayer only works on wi-fi - O2 said they'd block it if the BBC didn't).
Phazer