Our school used to block ustream.tv because it has a chat feature and we can't have students chatting in class, can we? The block came off when the headmaster discovered the Hummingbird nest cam and wanted our students to see the live hatching of the eggs.
This, Skype, and other distruptive technologies can be both positive and negative in the classroom. If we insist on blocking all disruptive technologies, we lose any benefit we may gain from them. Should we ban pencil and paper because students use these to pass notes in the class? The solution has to be classroom management, not blocking everything that can be disruptive. Yes, we use Skype to connect our language students to native language speakers around the world. We have had students spending a term abroad who visit their classrooms back home via Skype. And yes, Skype is a problem for our teachers and they have to deal with the problems.