This is apparently security related and not Microsoft trying to block off third parties, but here's a funny about Microsoft complaining about others dominance.

See the article here:

http://news.com.com/2100-1032_3-5066412.html?tag=fd_top

Yep, it's official... neither Trillian nor MSN can garuntee that Trillian users will be able to connect to MSN after October 15th... Interesting when you consider that Trillian's paid versions come with a "NO REFUNDS for any reason, including failure to connect" policy! I hope that any individuals who are considering the purchase of Trillian's new release are aware of the risk that they are taking... What's more, is that Yahoo and ICQ will likely and logically follow suit and may expect monetary compensation from third party IM clients as well. A failure to connect to 2-3 clients could render Trillian useless...

Add to the above, the fact that people who do not renew their Trillian subscriptions will end up paying less for the same product over the long run, than those who do keep paying to renew their subscriptions/memberships and you can see how Trillian's pay scale works against it... And speaking of working against itself... Trillian boasts over 11 million downloads and Microsoft is considering third party IM clients individually and is assessing it's likely user base as a defining criteria... The more Trillian users there are the more it may cost Trillian's developers to maintain the ability to connect to MSN and ironically the more likey it will be that Cerulean Studios may find it necessary to decline to accept Microsofts terms and conditions for the right to connect and that could leave its users out in the cold...