Check
here for more offical, legal, take-it-to-the-bank details on bumpers. If you can come up with a question that hasn't been asked the GDC usually responds to questions in a couple days. But please make sure you read all the questions that have already been asked, the poor GDC people must be getting a bit weary of having the same question asked over and over again. (As you will see when you read the answers.)
Basically, however... don't get "creative" with the bumpers.
Jason