This page on Quora about Skype is quite old (two years, which in the Internet age is a lot!), but wasn't linked much and wasn't linked by myself although I did blog about Delphi and Skype several times. Came to my attention last month, and now that I'm doing my new year email cleaning up, find it is still worth sharing.
Original link on http://www.quora.com/Skype/What-programming-language-was-Skype-originally-written-in?mid=55546 . I think it is very relevant for Delphi, even if the decision appears somewhat casual.