If a WordCamp provides badge images, then I've always just used a Text widget to display them in a sidebar or similar. Not all of them make badges, but they should, really. I've talked to people at WordCamps that only found out about their local camp happening by seeing a badge on somebody's site.

A plugin for this is easy enough, if there was a central place to have them. Maybe we should make a badge generator thingy on wordcamp.org or something similar. Make it easier for WordCamps to not just have them for others, but to actually create them too.