Ok, the longer answer
I don't think there will be a sl6, and if there is then it won't be a major upgrade.
Right now on win8 the future is winrt for apps, wpf for full fat desktop apps. Delivery via browser is a toss up between silverlight or html5. I know which I'd choose...
Fast forward a few years and winrt gets more mature you can see that it will displace wpf. Microsoft is betting the bank on HTML for browser delivery as they know a plugin based environment isn't going to be fotm down the road.