Nearly two years ago I asked Where are the Silverlight developer tools for Mac?. Well we have the answer, there is a plugin for Eclipse called eclipes4SL which allows Mac developers to write Silverlight applications.
I am not vouching for these tools, I have installed them, but I don't know enough about the specifics of Silverlight (yet...) to make a claim either way, but the world has changed. With development tools in any form on the Mac, Silverlight as a long term cross-platform solution just became real.
I don't know if the Mac dev tools were out before Silverlight 3 was announced on Wed. March 18 at MIX09, but I found out about them through Tim Heuer's guide to Silverlight 3 features. Silverlight 3 addresses some more of my concerns for the environment. Native H.264 video/ AAC audio is included. Silverlight 3 applications can now run outside the browser, probably to compete with Adobe Air.
I recently started thinking about using Silverlight for a redesign of one of the day jobs primary .NET apps. The Silverlight 3 announcements, and I haven't even begun to process the actual development enhancements, make evaluating Silverlight an urgent day job task. The mental calculus also got a lot easier between Adobe Air and Silverlight with the alarming resource usage of a couple popular Adobe Air apps, Twhirl and TweetDeck, documented in this tweet and this one.