This is the new Gamertag Card feature available with the Xbox.com relauch of My Xbox. Basically, you can place an IFrame chunk into your blog, website, forums signature or whatever and wherever you feel the need. I suppose after the Xbox 360 launch it will be more meaningful, but I suppose it doesn't hurt to get on board now. You can find complete instructions on how to do this at Xbox.com under the My Xbox / My Gamer Card section.
Update: Apparently some blog readers don't support the IFrame inserted content. To see what I'm talking about, please visit the site directly using the link that's available in most readers. (Thanks to Jason Bock for pointing this out.)