As originally reported on the Neowin forums, Windows Live Messenger blocks sending YouTube links to other Windows IM users.
Speculation has grown throughout the web as to why this is. Monopolistic practices on Microsoft's part, or something else?
Today I discussed the issue with Rafael Rivera (maker of the Vista uxtheme patches on WithinWindows.com) who pointed out that a lot of sites are blocked, not just YouTube. For instance, deviantART, which doesn't compete with Microsoft in any way, is also blocked.
Our conclusion is that it's probably an unintentional, but stupid, mistake by Microsoft. But what do you think?