Booting up your Xbox 360 today will result in an automatically downloaded update. Don't be alarmed, it's normal and it happens to a lot of people.
The download is the much-anticipated (though not quite as epic as expected) dashboard update, which includes a variety of usability improvements to Xbox Live. Here are some of the highlights:
Netflix Parties and Instant-Watch
New Achievement Browsing
Improved Live Party Support
It's not a bad update and it definitely appears to make the overall browsing experience smoother than it was before (with fewer 20 second lags when pulling up your friends list). I'd say you should check it out, but since you basically have to if you ever want to connect to Xbox Live again, there's not much of an option.