It's all on http://www.maemo.org.

It states that a beta version of Internet Tablet OS 2006 Edition is available for download from http://www.maemo.org/downloads/nokia_770, but when going there it's nothing more than the 2005 Edition.

Maybe it will be up later today..

Edit: The beta image is available for download now. It should be noted however that this image is primarily intended for developers and beta testers, not for everyday users, and it doesn't include VOIP/IM functionality.

Edit 2: Seems like it in fact includes VOIP/IM. I haven't had a chance to test the beta myself yet, but this is what I've heard.