According to a post by a Microsoft employee this week on the Windows Messenger 5 newsgroups Windows Messenger 5 Beta 2 is due to be released this week by the team.
Maybe more we"ll see some big changes but I"d very much doubt it, it seems at the moment that Windows Messenger is clearly a corporate version of the two Instant Messaging applications or what some would call a "slim lined" version out of the two, for those who want complete Windows XP compatability without the frilly bits of MSN Messenger 6.
Whatever happens more details are likely to come near the end of this week.