Just downloaded the latest preview. I'm running Windows Vista in Unity mode, the Windows taskbar is at the top. When right-clicking on a program in the task bar that is running to get the menu, I see it appear but then disappears again.
I have also seen some other issues with focus in Unity. For example, running 2 IE windows. I click on the 2nd one and focus instead goes to the 1st one. After trying for multiple times it sometimes then sticks and I get the 2nd one to show. I see the other one coming up but then being pushed back again by the 1st one. It seems to be always going to whichever had the focus last time and not specifically to the one I want the focus to go to.