I'm installing this as i'm posting this... though the installation is waiting for me to close IE7-Vista to proceed.
Anyhow, the link to the final version is here. http://download.microsoft.com/download/3/e/0/3e0467fc-a26f-4001-b352-cd1a274f957b/EN/Install_Messenger.exe
Update: This version does not have Yahoo! Messenger Integration yet, this long-waited feature will be available in coming updates.