IBM has announced that they will soon be porting Sametime server to Linux. So with the release of the Sametime Linux client (due in Sept with Sametime 7.5 and now in beta) and then in the first half of next year, the release of Sametime server on Linux enterprises will be able to adopt a pure linux instant messaging environment if they so choose. I am not aware of any other vendor that has offered IM support on Linux. Choice is such a great thing and definately puts IBM ahead of its competitors.

http://www.arnnet.com.au/index.php/id;1174295407;fp;8;fpid;0
http://edbrill.com/ebrill/edbrill.nsf/dx/sametime-server-for-linux-being-announced-today