Well it's funny you should mention that. An IM is currently in pre-alpha testing.
The NSIM isn't intended to replace AIM or MSN, it is simply a client interface to the online messaging system. Basically, it just makes it really easy for you to check your messages without having to open IE and log in. It docks in your system tray, and you can leave it logged on, so while you're doing whatever, you can open it up and see your messages in near-real-time.
The main advantage is that you don't have to sit there refreshing the page, it does it for you. Also, you can leave it on so you don't have to log into NS every time you want to check your messages.
It'll probably be about a month before we open it up for Beta testing.
Matt