For T-Mobile, it is ok to use WiFi in the US to offload voice traffic (sometimes without informing the customers), but in the UK it refuses to interconnect with Truphone and in Germany it altogether seeks a ban on VoIP over iPhone, whether the call is made over 3g broadband or WiFi. That sounds like more than double standards.

The German court has unfortunately sided with T-Mobile this time. In the UK, it did not have much luck though.

Mobile VoIP is actually not allowed fully in the US. You cannot use 3g mobile broadband connection to make a VoIP call. The wireless carriers are allowed to block such use. Only VoIP-over-WiFi is allowed. Indeed Apple has also sided with the wireless carriers in the US. It is allowing VoIP on its iPhone handset only as long as the call goes over WiFi.