AnandTech.com has an interesting overview article. It puts lots of information released over the last few weeks in one nice article with lots of own pictures. It talks not only about the hadrware with:

-The Xbox360 Hadrware and Design where Microsoft told AndaTech the Xbox360 will be quieter than the current Xbox and that the backside design isn't final yet (outputs ports).
-The Xbox Wireless Controller that will be lighter than the current S-Controller and with a new connector on the backside (Microsoft is being very quiet about it atm).
- The Removable HDD adapter that was still empty in the mockup AndaTech got to see but that will probably be a custom interface to a 2.5" HDD.

But also about the new Xbox 360 Dashboard with:
-The new Xbox Live with the 'invite' pop-ups that can display in the middle of a game - AnandTech has screenshots of this.
-The GamerCard that displays the player's reputation (measured in stars), the GamerScore (counter of all achievements across all on- and offline games), the Zone (defines what sort of player you are) and an avatar (the can also be a picture taken with the Xbox360 camera)
-The Xbox Live Messaging that will not be compatible with 'e-mail' as MS didn't want spam on the Xbox 360. You'll also be able to keep a friends list and see what they are doing or which game they are playing.
-The Xbox Live MarketPlace where you will be able to download or purchase new content (demos, themes, levels, etc.)
-Xbox Live Arcade with 'old school' games. The article also talks about the different devices you'll be able to connect with the Xbox 360 like digital cameras, mp3player, Ipods and also about Windows MCE PCs where the Xbox360 will act as a Media Center Extender.

Xbox 360 Interface in Action:
BetaNews.com posted a 2nd article about the Xbox 360 Interface/Dashboard (Video). The first article already had some interesting info and pictures of the Xbox360 dashboard, but now you'll also get videos of Xbox Live and Xbox Live Messaging!