The Skype Protocol Was Reverse-Engineered
This researcher has successfully reverse-engineered many of pieces of the Skype protocol (version 1.4 of the spec) for RC4 and arithmetic compression, etc. He also has a "send message to Skype" open-source code example. Version 1.4 of the protocol is slightly out-of-date right now.
For those interested in the Skype protocol, more details are available on skype-open-source.