I believe the current policy is strict for international flights (no bag without a passenger, must remove it if passenger misses flight), but looser on domestic flights. Not 100% sure that's true, but digging in some air-travel forums comes up with airline employees claiming the same thing as of late-2009.
There are lots of cases where baggage gets delayed or waylaid, and so the passenger ends up flying without their luggage. They then send that luggage onwards on the next flight — obviously without the passenger.
Yes, it would be hard to engineer that to happen, so it's not very useful to someone planning to do something nasty with it — but it does mean there can't be a strict no bag without passenger rule.
