Looks like Microsoft's recent upgrading of Messenger for Mac to version 6 didn't go down as smoothly as they would have liked. Microsoft's own Messenger newsgroup is just one of the places buzzing with people having problems logging in.
I found this out while struggling through this jungle of versions myself. Going from 5.1.1 to 6.0.0 locked me out of the service, with only a typically vague error message to guide me. To make matters worse, I was able to log in to the fancy new Windows Live Messenger on PC. Trashing my preferences and going back to previous versions did nothing, and Google wasn't much more helpful. That was around the time I stumbled on to the newsgroup.
In the end, my solution was simple enough - I changed my admittedly weak Passport password to something a little more robust and, voila, I was in.
Are any of you having similar problems? Let me know in the comments if so and what workarounds you've found.