It's a massive overhaul of the current dashboard...actually no...it's a totally new dashboard I've had a poke and a prod around various sites, and it does look quite funky (and yes - the Wii style avatar things are true, but they don't look that bad actually).

I'm really quite looking forward to it, hopefully it won't be ridiculously slow when it's finally released (I can think of a certain (read: current) release that can be painfully slow at times...)

I also found a link to what looks like a genuine (if somewhat small) gallery of images for the new dashboard.

ShackNews Gallery: http://www.shacknews.com/screenshots.x?gallery=10254&id=122358
Wired Blog Post: http://blog.wired.com/games/2008/07/pics-xbox-360s.html

Official Xbox.com Announcement: http://www.xbox.com/en-GB/E3-2008/nxe.htm