Adium just auto-updated to the latest version (1.5) and upon restart I was alerted that my SIPE Plugin was not supported on this architecture. Bummer.
I’m not sure, but it looks like I was running a 32bit version of Adium and the update pushed me into 64bit territory. I’m totally fine with that, but the SIPE plugin was 32bit.
Download the “SIPEAdiumPlugin.AdiumLibpurplePlugin(64bit).zip” file linked to above. Uninstall the old via the Adium Xtras Manager (From the Adium menu) and double-click the new plugin contained in the aforementioned .zip file. Voila, back to chatting on the corporate Microsoft Communicator/Lync network.
Update: Edited language to be more clear.
Update 2: This plugin will install in Adium 1.5.1, but will not work without running this command in the Terminal.
Update 3: I just updated to 1.5.2 this afternoon (July, 24, 2012). It appears this update undoes the work in update #2. If you view the Accounts tab in Adium Preferences you’ll see your SIPE account hang at ‘Connecting’. Close Adium and run the commands linked in update #2 to get thing working again.