T-Mobile bans VOIP from its flat-rate data services: "Such high speeds would seem to make the new data card ideal for applications such as Internet telephony and instant messaging. However, the fine print for Web'n'Walk Pro reveals that these are explicitly banned by T-Mobile, and any user caught running the applications risks expulsion from the network."

Well... I can't blame them for it, but it does rather mess them up as an example of operators becoming bit pipes.