Seems that Reenactor Fest last year was successful, and Reenactor Fest II is in the works for 2007.
This Convention cuts across many periods, drawing many reenactors. The site describes the convention like this:
What is Reenactor Fest? It is a convention for people interested in military history, as well as historical reenactors of all time periods, spanning from the days of the Roman Empire through WWII. It will be completely unique in this regard... a celebration of the hobby of historical reenacting as a whole. A chance to learn about other time periods, as well as your own!
I recommend going to this site, giving it a good look over. As a matter of fact, it might be worth a trip to the Windy City. I also recommend a trip to the site's Photo Gallery. Lots of neat stuff.