It’s not as backwards-compatible as most -webkit (or -moz) CSS stuff, but still usable. And, more importantly, it’s going to be very useful in web applications for the iPhone. (Which is obviously why they’re implementing it.)
P.S. Ok, the post concludes that it degrades gracefully if you place several background (or whatever) declarations one after another, but I can’t believe there isn’t a version of IE somewhere that screws up and switches everything to white if you do that.
In the meantime, why doesn’t anybody post screenshots? I don’t want to download the nightly.