This upgrade will happen in two phases. In the first phase, Windows XP or Windows Vista users will get a prompt encouraging them to upgrade. When you accept the upgrade, the new unified installer will launch and start upgrading Messenger. While its being installed, you also have option to download and install other Live applications available via Windows Live Unified Installer. If you haven't tried other Live applications, do it, I am sure you'll like it. And this phase may take couple of weeks to get the upgrade out to all users, and if you want to install it now , you can go to http://get.live.com/messenger/overview and download and install Messenger 8.5 right away.
After this optional upgrade has reached everyone, and those still running the beta version of Messenger 8.5, will receive a required upgrade to the final version. And in this phase, those still running beta will be blocked from logging in until he completes the upgrade.
A special note for those running Messenger 8.5 Beta on Windows XP 64-bit or Windows 2003 Server, the final version of Messenger 8.5 will not install or run on your OS. So these people will not receive the mandatory upgrade to the final version. However, these operating systems won’t be supported by future Messenger versions. So these people may stay on their current beta version, or to get to a more stable final version, its recommend that you uninstall Messenger 8.5 and go back to Messenger 8.1.
UPDATE: I too had a doubt and as LiveSide posts a note for Windows XP 64-bit and Windows 2003 server: "Messenger 8.5 itself works fine on these operating systems, it is the new Live installer that does not." Thanks Sunshine.
As per Jonathan Kay Attention Messenger team: Messenger 8.5 works on Windows Server/Windows XP x64 + petition
I'll update further as I check this out.