Thanks EvilSeph for the heads up
MSN Messenger 6.1 has been silently updated, although nothing has changed visually.
MSN Premium/Plus are both due to launch next month alongside a new MSN Hotmail, improved for spam and email organisation. MSN Premium is expected to ship with MSN Messenger 6.1. Plans for any 6.x updates are unknown at the moment and 7.0 is being developed internally.