personally i have troubles juggling msn and Wow and alt tabbing back and forth causes lag, also i miss alerts. i was just wondering if such an introduction would be possible, probably not, but it never hurts to take a shot.

Not possible. WoW is sandboxed, and has no access to the internet other to the game servers, and thus, neither do addons. Besides, even if someone was crazy enough to create hackcode, it would require to reload the UI to see updates, check if there are updates, and even to send messages.

If you don't want to alt-tab, then run the game in windowed mode.

While this couldn't realistically be implemented into WoW, there's no reason it can't be done using an overlay just as Fraps, XFire and PLAYXPERT do. In fact, PLAYXPERT's friends widget is multi-protocol and supports MSN.

I personally use Xfire everyday and will verify that it works great even for msn.