We haven't used rivets, but a quick search of the forums for "rivet frame" comes up with many related threads, such as this one
http://www.chiefdelphi.com/forums/sh...ht=rivet+frame
I can say that welding the kitbot frame significantly increases frame stiffness relative to bolts, and saves probably one or two pounds of bolts in the process. Rivets offer a signficant weight savings, too, I understand, and in this game saving a few ounces can make a big difference.
Jason