From Mary-Jo Foley’s All About Microsoft blog:

Several of my customer and partner contacts have told me they have heard from their own Microsoft sources over the past couple of weeks that Silverlight 5 is the last version of Silverlight that Microsoft will release. They said they are unsure whether there will be any service packs for it, and they are also not clear on how long Silverlight 5 will be supported by Microsoft.

Considering Silverlight was Microsoft’s answer to Adobe Flash, this should come as no surprise. With Flash not able to gain solid traction in the mobile space, and Microsoft throwing support behind HTML5, keeping Silverlight around makes little sense.