I just walked down to the bike rack at my office with a trusty 5 cent Bic pen (pilfered from the supply cabinet), and lo-and-behold came walking back upstairs with two Kryptonite locks; one brand new KryptoLok, one Bell U-Lock, and one very nice bike belonging to my friend in the cubicle next to me. One important thing to note if you're doing this on your own lock: When opening the lock, if you twist the pen the full 180 degrees of the cylinder then the cylinder gets locked into an open position. So be careful, just twist it about 90 degrees or so, till you feel it. Otherwise you'll end up with a bum lock that can only be closed again with a filed key.
Your U-Lock is not safe. From the blue: Any circular keyed lock, including (especially) all models of Kryptonite locks, can be compromised with a fucking Bic pen. See the video, try it for yourself.