It's probably been noted in a hundred other places by now, but the new corporate IM client (link to Trial Version, the Office website hasn't caught up yet) was just released.
I'm using it against the Microsoft LCS2005 implementation while signed in from home for some late, late case work. I really like being able to securely use corporate IM across the Internet, and stay signed into play-Messenger at the same time for socializing*.
It's a tad disconcerting to look at the status of a possible IM target and see the "Free at 2am" message (and makes me want more coffee) but it's so useful, especially when working across timezones.
LCS 2005 also provides support for third-party IM networks, which will be useful for communicating with customers, but I think I'll keep the friendlies confined mostly to MSN Messenger instead.