This blog pots of Lee is on Custom Commands in Silverlight and Prism 4.
Source: Lee's corner
Silverlight 4 includes commanding support for ButtonBase classes, but for anything else we would need to write a little bit of code.
There is a nice section here on how to extend this to other situations. Here is a quick sample on how to create command support for MouseLeftButtonDown on an UIElement. You can download the code from here.