Sure initially Silverlight will play nice with lots of platforms...
Just like Windows NT ran on Dec Alpha, MIPS, PowerPC and Intel computers, but as soon as Microsoft had achieved dominance in the workstation arena they cut out the other platforms and went back to their own system. Just like when then started late in the browser wars, they made sure they played nice long enough to get the majority of the market then they started enforcing their IE based standards on the web (the old "embrace and extend"). The same will happen with Silverlight.