Radio Netherlands has an interesting comment about T-Mobile/Netherlands not being opposed to Skype on the iPhone, while sister T-Mobile Germany is.
In the Netherlands, T-Mobile is the exclusive provider of iPhones. T-Mobile Nederland is not against the use of Skype on its network. The German branch of the company, however, is blocking its use by mobile phones, officially to prevent the network from becoming overburdened.
Interestingly, when I queried the USA folks who represent T-Mobile I received this response from a T-Mobile spokesperson:
"T-Mobile is supportive and working towards a more open wireless experience that invites innovation to our network and embraces customer choice and we are committed to enabling a broader set of devices and applications to run on the T-Mobile network. Our goal is to provide an optimal wireless experience for our entire customer base, and if it's determined that a customer's use of a third-party application may lessen this experience for our other customers, we will take the necessary precautions to protect and maintain an excellent customer experience on our network."
So until Skype is running on the T-Mobile handsets in the USA (meaning Android) in a big way, over 3G or other apps like Truphone and Fring, or even SlingBox, we won't know just how far T-Mobile will go here.