Do not be fooled. Silverlight is only partly cross platform.
MS says themselves Silverlight is not entirely cross-platform since some other platforms do not have the advance technology that MS windows has, so the experience will suffer.
Or in the words of the register:
"There is also a new trusted mode, which requires user approval, and enables local file access, COM automation, and cross-domain networking access. [..] More seriously, COM automation is a Windows-only feature, introducing differentiation between the Mac and Windows implementations." (http://www.theregister.co.uk/2009/11/20/silverlight_4_windows_bias/)
And MS themselves: "Microsoft is committed to providing full cross-platform and cross-browser support with Silverlight and to optimizing Silverlight to light up every platform on which it runs," "every platform on which it runs" == some (Mac OS X for instance) except Linux. The moonlight team is working on that, not MS. So MS will not care about compatibility with Linux. So it is not cross platform. It's crossplatformish. (http://arstechnica.com/microsoft/news/2009/11/lack-of-cross-platform-support-in-silverlight-4-explained.ars)
And from MSDN : "Trusted applications can integrate with native functionality that differs depending on the host operating system. For example, on Windows, trusted applications can interoperate with Automation APIs through the AutomationFactory class." (http://msdn.microsoft.com/en-us/library/cc838247%28VS.96%29.aspx)
Silverlight is the next IE6. Using it now, will cost you dearly later.