I was reading a thread about Commands in the WPF Disciples group and noticed a comment about being unable to initialize an attached property where the type was collection. The problem was unless you put in an explicit Collection object tag, the property...

From Dave Relyea's blog, Pixel Snapping in Silverlight as an attached behavior: http://blogs.msdn.com/devdave/archive/2008/06/22/Using-an-Attached-DependencyProperty-to-Implement-Pixel-Snapping-as-an-Attached-Behavior.aspx