I figure this will be the first of several Internet related updates, but it came as a nice surprise today when I read about it. I downloaded the update tonight and it just adds YouTube to your list of options on the main menu, and is really easy to use. All videos open up to full screen, which is neat to see on a large TV even if the video quality isn't great. You can browse the same ways you can on the website (newest, rating, this week, today, etc), and when a video finishes you are taken to a related video page.
Anyway, just a nice little add-on that I see getting a decent amount of use at my house.
I just bought a Mac Mini, and I've found it painlessly easy to turn it into a Media Center PC-especially once I downloaded and installed Apple's Front Row program. All I need to do now is add a USB IR remote to the system, and I'm set.