Yahoo Messenger and MSN (Windows Live) Messenger work together

The proposed interoperability between Yahoo! Messenger and MSN's Windows Live Messenger finally went live today.  Now when you use one, you can add your buddies from the other, and talk back and forth.  This gives Yahoo! and Microsoft a comparable userbase (around 44%) to AOL Instant Messenger, which has itself about 56% of the market.  The numbers aren't perfect, since there are other messenger programs out there (Google Chat), but most people use more than one at this point.  When Google Chat becomes interoperable with AIM, then it will become much more significant in the market.