I have just switched to the latest version of Moonlight, the port of Microsoft Silverlight on Linux, and the least I can say is that things have significantly progressed since the last time I tried.
The only problem seems to reside in the Silverlight detection algorithm used by most of the sites. Most of them give me the Redmond finger:
... while Moonlight is there and ready to chew XAML and render it! Too bad, I would like to see more of these sites and keep track on how this runner up technology will compete with the ones already in place.
At the end of the day, I have no doubt Silverlight/Moonlight can become a credible alternative to the current RIA browser-side technologies. My main concern is that sloppy developers may create Silverlight-powered sites that will only work with a limited set of platforms (browsers, OSes), which would be a regression from the current state of the web nation.