if you move the WMP11's window to the upper side of your screen taking from
the bottom panel, the window will return down and appear completely
transparent.

Windows Vista SP1

reproduction steps:
1. open WMP11, not maximized.
2. in the bottom panel, click at the right of playback controls and move the
window to the upper side of your screen until the window goes out of screen
3. release the mouse button
... and the window will return down and appear completely transparent, move
the window and return normal.