In addition to this photo of a bike where the police rider used their handcuffs as bicycle lock — previously posted on Street Use — here is another one with the same idea (found on this guy’s blog).
I’d advise against handcuffs… they are an incredibly easy lock to pick. Seriously, grab something thin and stiff (like a hairpin), stick it in the lock at a sharp angle, and twirl it in a full circle. There’s good odds you’ll release the lock.
Handcuffs are made for keeping people restrained while under supervision. They are not made to prevent casual theft.
i’ve been using handcuffs for a while now and can’t talk them up enough. they’re small and easy to fit into your packet – no need for a bulky chain.
here’s a pic of it in use here: http://www.markrobertallen.com/2007/03/out-and-about-round-valley.html#links