In WPF, the MediaElement is simple and easy, but there are times when one wants to do more with video (webcams, pixel manipulation, etc.).  This usually requires dropping outside of WPF into unmanaged code or at least referencing unmanaged dlls, all which can get tricky, suffice to say.  I wanted to point out two great WPF video samples that recently came online which abstract out some of this complexity and let you get to the task at hand.

http://www.codeplex.com/VideoRendererElement: With this project, by Jeremiah Morrill,  "a developer can update a pixel buffer or video media sample and have it render in WPF space at MediaElement speed. It is compatible with GDI, DirectShow and direct pixel updates."  The nice part is that he's abstracted all the DirectShow code, so you can work with this project and stay in safe .NET land.

http://blogs.msdn.com/uberdemo/archive/2008/03/27/capturing-a-webcam-stream-to-a-wmv-file-from-within-a-wpf-application.aspx: With this project, you have probably the best class I've seen out there for capturing a webcam with WPF.  Very clever how he uses the Windows Media Encoder Series 9 for the capture. His write up is very thorough.