I enjoyed this post by Whitney Hess, a user experience designer, about Essentials to Designing a Conference. She includes photos of the badges with the designs she likes best. Some tips? - make sure they don't flip around annoyingly, maybe make sure they aren't one-sided, and leave room to write on the badge.

Very interesting perspective - she's a designer of user experience, but a participant in conferences lending her perspective to the overall design of an event.