The setup is simple:
Make sure you’re running Adium 1.5
Download this plugin: http://db.tt/2yx5wYNt
Install the plugin and restart Adium
Add the new account using Office Communicator for the protocol
It doesn’t do everything (no video chat or screen sharing) – but it’s a huge step in the right direction.
[Update 5/4/12]: One thing Adium doesn’t do (but lync does) is set your status based off your calendar. I’ve addressed this via some AppleScript, as detailed here.
[Update 5/16/12]: The latest beta of Adium (1.5.1b1) uses a different version of libpurple, so the plugin doesn’t work. A quick workaround is to run the following commands in Terminal:
ln -s 0.10.5 0.10.0
Plugin is also available on V:\IT Share\IM Clients\MAC\Adium + SIPE Plugin\