Chances are, those of you who installed the Xbox Live system update yesterday haven't noticed anything different. The dashboard itself hasn't changed at all in design, but Larry "Major Nelson" Hyrb has confirmed that it updated the bits and bobs that we don't see, "The system update contains behind the scenes improvements and prep for future enhancements," said Hyrb.
"Future enhancements" perhaps suggests that we'll be seeing mroe updates soon. Even though we've been told it's only behind-the-scenes stuff that's been changed, I spotted something else. If you scroll over to the "Social" tab, you should see a button that says "Avatar Store," which was never previously there. The link takes you straight to the avatar marketplace, so you no longer have to click on your profile and press "customize avatar" to get taken to the marketplace. Of course, it's a very minor change—I don't think any of us were too bothered by if beforehand.