I have this odd behavior on my MBP after installing Lion. I'll be replying to a message in Mail or moving a window in finder ... and suddenly the window disappears. Sometimes I find it down at the bottom left of the screen, with only the title bar showing. Other times, it will "walk" to the bottom of the screen on its own ... a "stutter step at a time," while I try in vain to stop it.
In Mail, this will tend to happen if I start typing in the reply before the Genie has openened the message all the way. In the Finder, it seems to happen if I move a window too quickly to the edge of a screen or across to my second monitor.
It's slightly maddening, and a real waste of time as I watch helplessly as windows march their way to the bottom of the screen. Any help is much appreciated.