"We've given YouTube a refresh. When you login, you'll be greeted with recent activity from your subscriptions. You can also browse and add new Channels to your homepage. Connect with Google+ and Facebook to share your favorites with friends. There's plenty to discover, so login and give it a spin."
There's also an article about the new interface for channels.
This new look and feel makes it easier for viewers to to find and watch content. It also makes it easier for Channel creators to organize and showcase exactly what they want. We incorporated your Feedback from the "Cosmic Panda" Channels and Watch experiment, and are excited to present:
* A more streamlined and consistent design
* An easier way to find and view videos
* New, more flexible layouts for featured content, and
* A way to keep your audience engaged even when you don't have new uploads
It's interesting to notice that the new homepage and the new channels are built around feed views and activity streaming to make YouTube even more social.
The new channels are less customizable: you can only choose an avatar, a background image, a background color and a template (creator, blogger, network, everything).
Update: The new interface is available for everyone.