Check out the demo.
From what I've seen so far after kicking the tires, the managed silverlight platform seems new and somewhat incomplete, but provides just enough basic build material to stay interesting - certainly no accusations of over-engineering (yet).
One immediately apparent frustration is that the mac hosts are at least an order of magnitude slower than the windows hosts when it comes to rendering - I hope this is only a temporary thing. Once we have some parity here, it would be *great* to get some cross-platform low-level 3d primitives - taking advantage of the hardware of course.
The wpf library turned out to be far more stripped-down than I first imagined. Again, I think this is actually a good thing - it's not hard to completely wrap your head around the entire mini-wpf control set. I did find myself looking for better brushes, though - it would have been nice to have the DrawingBrush or the VisualBrush from mega-wpf.
Also there are no built-in UI thread-marshalling functions even though the threading primitives are tantalizingly exposed. You basically have to rig up a bootleg ui-pump yourself, using the Storyboard object, which conveniently raises events on the correct thread. Again, not something you'd want to do on a regular basis, but it works for now.
It will be interesting to see what types of .net applications start making the jump into this very "applet-like" environment. Sorry of course I meant "Rich and Interactive" environment...