OMG, Can anyone here post without a clue??
Silverlight is the runtime (think: JVM) of course you can't have a windows JVM and install it on Linux. That's stupid.
It's not the Silverlight runtime thats cross platform, it's whatever apps you build for that platform that makes it portable.
Let me break it down for you:
Silverlight Runtime ~= JRE
Silverlight Runtime ~= Moonlight runtime
These runtimes only runs on the platform it was built for, i.e. you can't run a windows JRE, on OS X, an OS X jre on Linux, etc, etc.
Silverlight .xap ~= Java applet .jar
Silverlight .xap ~= Flash .swf
The exact same .xap will run *NOW* on Silverlight+Windows/IE/FF/Chrome or Silverlight+OSX/Safari/FF
That my friend is what we call cross-platform, the same .xap I developed with VS.NET (or eclipse: www.eclipse4sl.org) will run unmodified now on OSX and is likely to run on Moonlight+Linux.
Once Moonlight is feature complete and all tests pass, the exact same .xap will run on Moonlight+whatever platform moonlight supports.