I have been playing around with using track matte for transitions. In doing so I've noticed that the Wingdings font (and relatives) provide a multitude of shapes that can be used for transitions. Here are a couple that I've put together in PE.
[wmvvideo]http://muvipix.com/cpg/albums/userpics/10215/wingdings.wmv[/wmvvideo]
Hope it inspires someone...
Chris.