I can confirm that this temporary solution works for me every time as well. I'm also frustrated that this problem has yet to be fixed, since it has been present for me since the early beta. After testing on multiple systems, my best guess is that it is related to the video drivers, since only those computers with certain NVIDIA gfx cards are what I've seen this on. Certain driver versions appear to work, but not the latest one from the NVIDIA website. Clearly, the screensaver is involved, when looking at antient's temporary solution.
Interestingly, once the bar has disappeared, if you wait for the screensaver to turn on again, then move the mouse, the bar reappears. This is esentially the same as previewing the screensaver.
Has anyone figured out a real solution besides reverting? It's the only real thing holding back my Win7 love.