The honorable Major Nelson just posted that someone created a Vista Gadget that will display the GamerTag information for your friends on XBox Live. Well, for those who don't know, if I'm not mistaken, Vista Gadgets are generally written in Silverlight (Please correct me if someone knows better).
But, how to get that information? Well, Silverlight doesn't have Databinding yet, but WPF does. If you check out the Major's post, he points you to a Microsoft Evangelist by the name of Adam Kinney who wrote the gadget. Following him, you end up with Duncan Mackenzie who talks about how to connect to his services and get GamerTag info.
Here, to make things easier, open up Blend, and try this:
Create a new project, and then click +XML in the Data Pane. The dialog will come up asking for DataSource info. Paste this into the URL for the XML Data:
Now, check out the Data pane. You'll see XboxInfoDS, which you can expand to see a bunch of data fields. If you drag out the "Gamertag (string)" onto the design surface and create a Label, it will proudly display "LordSaphon" (since that was the Gamertag you supplied in the URL, and yes, I am LordSaphon).
You can go ahead and drag the GamerScore, ReputationImageUrl and so on out to the artboard and you'll see those fields displayed as well. Please, don't laugh at my GamerScore... I know it's not huge :).
Remember that any field that is an image Url should probably be bound to an Image control. You can create your own WPF Gamertag application. It also shouldn't be too difficult to modify the Source property of the DataSource, so you can dynamically switch which user you want to display!
--Dante (LordSaphon on XBox Live)
PS: The service I'm using for getting the XBox Live information is on a webpage not affiliated with Microsoft (as far as I know), so if that information changes, the results you get are beyond my control. This information is provided "as-is", so don't hold me or Microsoft responsible if you get anything unexpected from the site.
Duncan Mackenzie: http://duncanmackenzie.net/blog/put-up-a-rest-api-for-xbox-gamertag-data/default.aspx
Adam Kinney: http://adamkinney.com/blog/272/default.aspx
Major Nelson: http://majornelson.com/archive/2007/10/25/xbox-friends-watch-a-sidebar-gadget-using-silverlight.aspx