After toying a little with the new Silverlight 4 camera features, and being really disappointed that these were not included in WPF, I looked at the Silverlight assemblies and found that they more or less delegate all work to agcore.dll.
Do you think it is a good idea to package agcore.dll with my WPF application, and copy/paste or rewrite all classes concerning cameras to WPF, to get these nice features there?
That particular approach isn't likely to work, unfortunately. There's a lot more to getting C# code to work with Silverlight than just referencing the appropriate DLL. Silverlight is actually a completely different implementation of the CLR, so you can't just call into agcore.dll from a WPF app and have it run.
If you need Silverlight features in your app, I can think of two ways to get them: (1) you could conceivably host a completely separate instance of Silverlight inside a browser control in your WPF app, though communication with that Silverlight instance becomes complicated (though not impossible); or (2) you could write your entire app in Silverlight OOB (Out-of-browser)/trusted mode.
You can try VideoCaptureElement in my WPF MediaKit. It will show a webcam in WPF. It does have a different API than the Silverlight webcam API, but it should have just as many features.