I use Messenger Plus! Live for one reason.  Tabbed chat windows, and auto-file-download-accept-thingy.  Ok, 2 reasons...  But anyways, when tabbed chat is enabled, I enable it for all chats automatically, etc...  And all the chats are grouped just fine, however, when I get a new message the button on the task bar doesn't flash.  Sometimes, the actual tabs won't flash either, say when I'm typing to one person, and some one else that I already have a tab for sends a message, it won't blink.  So I have to constantly cycle through all the tabs to see if I missed anything, which kinda defeats the purpose.  This happens on any Vista system that I use, with the latest Messenger Live and latest Messenger Plus builds.  Even earlier versions of Messenger Live cause the same problems in Vista.
Windows Vista 32 bit - tested on Ultimate, Business, Enterprise.
Thanks for the awesome product, sadly I won't be using it until this works again.  
