From the blog post entitled The i'm Initiative and new secret emoticon on the Windows Live Messenger team's blog we learn

Not everyone has the financial ability to give money to the causes they care about. That is where the i'm Initiative steps in - it enables Windows Live Messenger users to make a difference by directing a portion of Messenger's advertising revenue to a cause of their choosing.
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Wonderful! How does it work?

1. Use Messenger 8.1
2. Add the i'm emoticon to your display name by entering the code of the cause you would like to support 
3. Send and receive IMs
4. A portion of the advertising revenue generated by your usage of Messenger will be donated to your cause. So the more IMs you send and receive the more money will be donated to your cause.

How does Messenger even generate revenue\money anyway?

Windows Live Messenger is a free service to users. We do include advertisements in the client that help pay for the service and our salaries. With the i'm Initiative you get to decide where a portion of the revenue goes.

The list of codes to create the emoticon are listed in the blog post. I'm using *9mil in my IM handle. This trend of tying charitable donations to the usage of Windows Live services is interesting. It's kinda cool for our users to feel like they are contributing to the betterment of the world simply by using our software the same way they have every day. Good stuff.