I have never had anything that was bolted on the bike stolen off of it and that was locking up in some pretty bad areas for all day. The folder is a good idea if you are that paranoid. Check out the Brompton it is hands down the smallest and quickest fold. Bike Friday Tikit comes in a very close second. I use a vintage Dahon Classic III, folded and in a suitable bag it is smaller than most diaper bags and many purses and backpacks that I see.
Aaron