One of the focal points for Windows Live Messenger 8.1 is compatibility with Windows Vista, as there were many reported issues with running Windows Live Messenger 8.0 on Vista.
Having signed into Windows Live Messenger 8.0.0812 on Windows Vista just before, I was prompted with an optional update. Not expecting an update from 8.0 to 8.1 beta, I did some testing, and have come to the conclusion that the Windows Live Messenger team is confident version 8.1.0168 brings enough fixes to warrant an automatic update.
As a side note, the recent update to the Windows Live Messenger 8.1 beta has been made mandatory for those using the previous 106 build by the auto updater.
I have put up a small script in the sandbox which shows two alternate CVR commands sent to the messenger server, the only difference being the version of Windows (5.1 being XP, and 6.0 being Vista).