A new update to Windows Live Messenger Version 2009 (build 14.0.8064.0206) will be offered over the next month as an optional upgrade, according to the Inside Windows Live Messenger blog, or you can upgrade to the latest version now, using the Windows Live Essentials installer.  No word  as to what changes have been made to the newest version (the current build of 2009 is 14.0.8050.1202).

Update After installing the update (running Win7 Beta), not only Messenger, but Mail, Photo Gallery, and Writer are also updated to 8064.0206.  A quick check shows that Win 7 still treats Messenger in the same "annoying" way, so it's back to running in Vista compatibility mode (see #28, thanks Tim!), at least for now.  Let us know in the comments if you find any changes.