Well, apparently the beta of messenger:mac finally has AV support. Though I must say, in terms of features the product is very disappointing. Adium is far ahead, but I experience some server errors once in a while (Adium claims it's because of microsoft's servers). Last time I tried messenger: mac I realized that it didn't even have any offline messaging. I also think that the "drawer" concept for the display picture wasn't implemented well. No tabs for chats, and when I moved it between spaces it was buggy. A very feature limited product especially when you compare it to Adium.
Looking at this in a more positive light, Adium doesn't have very good A/V implementation, I hope that this kicks starts A/V conferencing on Adium, even though I mainly use skype for my A/V conferencing needs. Another thing I wish Adium had was drawing support.