Skype over 3G in UK
"... allows US users to piggyback onto 3G networks when making calls over Skype. Previously this facility, which users outside the US would take for granted..."
Here in the UK, Tesco mobile (using the O2 network) T&C's specifically state that using the mobile data network to send VOIP data is not allowed and if they detect it then they will take measures to punish you (my wording). On the bright side, they don't care about tethering since they go by the amount of data you transfer and charge you if you go above your limit (500MB a month on my contract).