If your start menu is open when switching desktops using the Window Key + Number Pad key-combo, the shadow from the start menu remains as an artifact on the desktop. This occurs in both Vista Aero mode and Vista Basic mode. The problem does not appear to exist when switching desktops using the Window Key + Z method of switching desktops (which seems to only be available when using Aero). The only way (that I have found) to clear the artifact is to log out and back in. This issue has been observed in beta 0.2 and beta 0.3. I'm sorry I didn't mention it sooner, I just now found the Issue Tracker :-)
In the interest of duplicating this problem, here are the precise key-presses and behaviors that have been observed to manifest this problem:
Press and release the Window Key
Press and release Window Key + Number Pad to switch to another desktop
- Start menu closes but shadow remains
Press and release Windows Key + Number Pad to switch to THE PREVIOUS DESKTOP
- Start menu re-opens like it was when you were last on that desktop. When the start menu is closed, the shadow is darker (due to multiple layering of an alpha-blended shadow). In essence, there are now two shadow artifacts stacked on top of one another.
I have attached a screenshot that depicts the shadow.