Microsoft has released Native Extensions for Silverlight, a set of libraries which enable access to Windows 7 features including taskbar Jump Lists; access to attached devices including webcams, cameras and phones; the sensor API for accelerometer support; and even the ability to intercept Windows messages. The ability to intercept Windows messages allows lots of interesting hacks as veteran Visual Basic developers will recall; it was one of the tricks used to overcome limitations in early versions of VB.
The native extensions are only available to out of browser applications running outside the sandbox; the user must consent to trust such applications. Silverlight 4 already had the ability to use COM automation. These new extensions simply build on this existing feature, providing COM automation wrappers for these Windows 7 APIs.
What this means though is that Silverlight developers can create applications that integrate deeply with the Windows 7 desktop and local hardware.
Another way of looking at this is that the subset of Windows applications that can be implemented in Silverlight rather than the full .NET Framework has now increased. It lends some support to the theory which I considered here, that a future version of Silverlight will be the application platform for the Windows 8 app store and for mobile devices running Windows 8. This is speculation though; Microsoft has not said much publicly on the subject. Silverlight is well suited to an app store since installation is easy, updates are near-automatic, and apps are isolated from the rest of the operating system.
The native extensions are Windows 7 only. Forget the Mac, these things do not even work on Windows XP. They only apply to trusted out of browser applications though. Silverlight running in the browser still has similar features on Windows and Mac.