I found a Silverlight example in a previous forum post
that I was able to modify.
But the example there uses VisualStateManager, VisualStateGroup, and VisualState which appears to be a .NET 4 thing, and I'm at 3.5.
Instead I used ControlTemplate.Triggers, modeled after the RadTileViewItem default style I extracted using Expression Blend.
I hope this helps someone else.