I will explain the problem in simple terms that illustrate the limitation I'm running into.
I created a new blend project. I set the window's background to Red. I then add a button. I create a timeline that is triggered to run when the button is clicked. I create a keyframe at start and a keyframe half a second later that changes the background of the window to Blue. I run the program and click the button and the background fades from red to blue.
I do this again, but this time I make Red a brush resource and blue a brush resource. I set the window's background color to the red resource. On my timeline I try to make it so that the first keyframe references the red resource and the second keyframe references the blue resources. Blend (and apparantly WPF) doesn't let you do this.
Unless I'm missing something this seems to really limit your options when using defined resources instead of static colors for things. The practical application of this is that I've created a control that has a "title bar" section that uses some resources defined in a resource dictionary. I want the title bar to change color whenever gotfocus and lost focus happens on the control. I want those colors defined as resources so I can skin my application. In order to do this it looks like I have to figure something out in code (and probably not have any kind of animated fading between the two colors) instead of being able to create it in the XAML via blend.
Maybe someone out there can tell me that I'm overlooking something?