[UPDATE] 2009-03-13

Happy Friday the 13th. Just an update to this story - the SIPE plugin has been worked on by a number of great people including Gabriel Burt from Novell among a host of others. It is now stable and can be used with recent versions of Pidgin.

Check out their great work at http://sipe.sourceforge.net/. The source code can be downloaded at https://sourceforge.net/projects/sipe/files/

[End UPDATE]