Moonlight 1.0 Beta Available
The Moonlight beta installs easily, and works quite well (though some sites respond better than others, this seems to hold true with Silverlight in a native Windows environment as well). The few hiccups I encountered during installation had more to do with network congestion and user error than the application itself.
My Intrepid machine is very generic, and it seemed to require at least some of the development packages that would be necessary for building from source or Subversion -- even for the Firefox .xpi.
After these packages were installed and Firefox was restarted, I visited a few sites with Silverlight content. Each site prompted me to download and install the Microsoft supplied codecs (licensed specifically for Moonlight on desktop). The first three sites I visited would not download the codecs. There were no error messages, and I suspect network congestion is as likely a cause as any other. The fourth time, the codecs installed (quickly).
Once the codecs are installed, a page refresh is all that's needed for Moonlight to render Silverlight content. Though Moonlight's "official" testing sites (and those visited "in the wild") may not behave exactly as expected in every circumstance, what I tried was fairly responsive.
Moonlight is built on the Mono development framework. The Microsoft supplied codecs are not similarly licensed, and require agreeing to a click through EULA. The Moonlight project is preparing to wrap up the 1.0 release soon, and is looking forward to the 2.0 cycle, currently in the pre-alpha phase.