I saw 10.1 in person.
A friend who works for a major software company told me it was "a beta of Mac OS X that's coming out in August." Take that as you will.
These pictures are consistant with what I saw. I didn't get a build number because it was not in the "About this Mac" dialog box. The version simply read 10.1
I can say that I played with if for about 10 mins. It is much much much snappier then 10.0.4. Still not perfect or as fast at 9.1 but actually a usable operating system now.
I went through all the system prefs and the biggest difference was in the dock prefs. You could move the dock to the right and left sides in addition to the bottom of the screen.
The bottom, top, and middle choices worked sort of like a right, left, and center justify in a word processor tacking the dock to the chosen side. My appology I tried this when the dock was on the right side. I now wonder what would happen when changing this with the dock on the bottom.
There were three minimize effects. They were "Geinie, Scale, and Suck (needs better name) ." I guess apple does want to call the last the Suck Effect.

This lends credence to the idea of this being a beta. This release was given to at least one major software company that told their employes to load it for testing purposes.
In my excitement I forgot to check for DVD playback.