Bumping is a quick way of opening any door lock that uses the tumbler system (just about all of them except electronic card locks) - without the key. The video here shows an eleven year old opening a standard Kwikset brand door lock in about 3 seconds (WMV file). This video here is a news cast of a locksmith bumping open his own store in about 10 seconds.
The technique requires no skill, no training, and no tools other than a bump key and something to tap it with. The bump key can be made easily from any key that fits the type of lock by filing away parts of it. The way the technique works is you bump the pins quickly causing the tumblers to jump up, leaving a gap between the pins and tumblers (as in second picture). Twisting the key while the gap is present opens the lock. The technique opens the lock (without the correct key) as quickly as using the correct key would.
What is at jeopardy: your door locks, USPS and UPS post office boxes and padlocks. No damage is caused to the locks and no telltale evidence is left behind. Insurance companies can deny claims on the grounds that the door or lock was not locked.
The Kryptonite bicycle lock disaster (a plastic pen opens it in less than a minute) spurred the company to fix their locks quickly. Hopefully this information will spur door lock makers to fix theirs too.