Are you thinking of the server or the client side? For the latter, it is safe to assume that it's C/C++ (Media Center SW on Windows as well as Linux clients are developed in that language). For performance reasons, this probaly applies to the server side as well. Yet opther languages such as Java are also used, take a look at e.g. the "Programming Languages" section of
http://sourceforge.net/search/?type_of_search=soft&words=IPTV