The only thing you haven't mentioned is that Unix/Linux requires Moonlight to be installed in order to run Silverlight websites.
Silverlight runs on MAC OSX natively so you shouldn't have a problem there.
You can (and probably should) enable GPU acceleration - see this blog post for full instructions, but basically you need to include the line:
<param name="EnableGPUAcceleration" value="true" />
if using HTML and for ASP.NET add the attribute as follows:
<asp:Silverlight ID="Silverlight1" EnableGPUAcceleration="true" runat="server"
Width="100%" Height="100%" />
Windows phone 7 runs Silverlight applications so you can use the same code base - though you need to compile for it specifically.
It won't run on iOS at the moment - but Silverlight 5 is slated to run on iPhone and iPad. Source
I've just seen this post that claims that Mono (who do Moonlight) are bringing Silverlight to Android.