Personally, until I can find the real memory leak, I just disable window preview thumbnails. I don't like them anyway, and I usually can't have them since I don't use a compositing window manager most of the time.
(By the way, if anybody wants to disable them, right-click on the little "grip" to the side of the window list applet and choose Preferences in the menu that pops up. In the dialog that pops up, click Hide thumbnails on hover. Oh yeah, and then close that dialog!)