SJ brought a hand-assembled XO-3 prototype to the OLPC Community Summit in San Francisco (mass production only starts when someone puts in a large-scale order), and of course I tried to run Etoys on it. It's pre-installed (as on all XOs) and worked right out of the box.
I was able to paint and drag objects, but since there is no right-click support yet there was no halo to bring up a new viewer. Also, touch targets are rather small for my adult-sized hands, and since there are no hover events, some features don't work correctly (as we found out with the iPad version two years ago).
So more work is needed, as well as for the XO-4 which has a multitouch screen in addition to a keyboard and touchpad. Help welcome :)