I've blogged about it before, but seriously, the question has to be asked: if you're a developer with limited bandwidth to focus on mastering new technologies, why would you spend that time on Silverlight?
Not only is WP7 floundering, but now the news is out: the Metro version of IE 10 in Windows 8 won't support any plugins - including Silverlight:
Windows 8 will have two versions of Internet Explorer 10: a conventional browser that lives on the legacy desktop, and a new Metro-style, touch-friendly browser that lives in the Metro world. The second of these, the Metro browser, will not support any plugins. Whether Flash, Silverlight, or some custom business app, sites that need plugins will only be accessible in the non-touch, desktop-based browser.
Should one ever come across a page that needs a plugin, the Metro browser has a button to go to that page within the desktop browser. This yanks you out of the Metro experience and places you on the traditional desktop.
The rationale is a familiar one: plugin-based content shortens battery life, and comes with security, reliability, and privacy problems. Sites that currently depend on the capabilities provided by Flash or Silverlight should switch to HTML5.
If you're not on the HTML5 boat yet, I think the writing is on the wall: the Silverlight party is over (thank goodness).