In response to a question on MSDN, I crufted up a really simple SideBar gadget for Vista using WPF/E as the user interface. (It just shows a simple animation when first added to the bar). SideBar (and perhaps Live) Gadgets are likely to be a fun place to use WPF/E. You can download and install it here (assuming you already have WPF/E runtime installed:
WPFE Sample Gadget