I have no idea if it's true, but I once heard that if you don't urinate for a long time your mind can only focus on the urge to do that, so you can lie during a lie detector test and it won't register because you're thoughts are focusd on when you'll be able to get to a bathroom.

Here's some interesting sites about the poygraph:
http://antipolygraph.org
http://www.police-test.net/services.htm