I love love love this idea. You put a polaroid camera out by the guestbook and tell people to take pictures of themselves, stick it in the book and write a message. It's somewhat complicated so one person who did it said they had a bridesmaid or someone walking around and assisting the processes during the party (haha). However, I think it's a fantastic idea!
Of course, it's also kind of expensive to buy the book that is specifically designed for holding polaroids. Another tactic would be to buy a bunch of scrapbook photo corners (those sticky corners) that are meant to hold photos onto flat paper like in a scrapbook, and people can just stick their pictures on that way... also complicated, though, maybe even moreso. Like this:
Maybe we could, in advance, put squares on the pages so people just have to slide polaroids in...
Anyway, I would love to figure out how to make this happen. I think it could be really fun!