That's weird, MouseDown and MouseLeftButtonDown are not working here. Left-clicking on the button, test1 and test2 event handlers are not hit. There's no problem with the right mouse button.
<Button MouseDown="test1" MouseLeftButtonDown="test2" MouseRightButtonDown="test3" />
A bug .. Can somebody repro this

What's wrong with the left mouse button in beta2?
flyingsnow
Seraphic Raven
Gidon J
Never mind, I've found an explanation on msdn:
Working Around Event Suppression by Controls
One scenario where preview events are used is for input events and controls. Sometimes, the creator of the control might wish to suppress a certain event from originating from their control, perhaps to substitute a component-defined event that carries more information or implies a more specific behavior. For instance, a Windows Presentation Foundation (formerly code-named "Avalon") Button suppresses normal MouseLeftButtonDown bubbling events in favor of a Click event. If other elements towards the root of your application still wanted an opportunity that event, one alternative is to handle even routed events. But a simpler way is to change the routing direction you handle to be the Preview equivalent PreviewMouseLeftButtonDown.
ESTAN