Beta1?

As far as I know, the only other place you can find this out is via the support forums and the wp-testers mailing list. And since there's a link to the downoad on the forum post, I'm assuming it's a public beta.

If you're brave, roll it out. If you're not brave, you might want to wait for a second beta.

If you're looking for a list of changes since 1.5.x, you're going to have to wait a little while for the compilation. There are a lot. Most of the stuff is internal, stuff that you're not going to notice - and that stuff that goes unnoticed is why the system is great.