It happens... unless you make a specific effort to be noticed, the bus driver most likely will NOT remember who put the bike on the front of the bus... so if another bus rider grabs it, he's most likely not going to notice.
I've also heard directly of a couple of cases where someone put a nice bike on the front of the bus... another passenger put a cheap bike on the bus, then took the nice one when exiting the bus.
If the bus looks full, or if it's on a line that gets very full, I'll throw my cable lock on the bike and around the rack... adds about 10 seconds to de-boarding, but it's worth it for my peace of mind. Otherwise I make sure I stay at the front of the bus where I can keep an eye on my bike!
I also make it a habit to talk to the bus drivers... they always remember me! I've gone on a bus route that I've only ridden once before, and had the bus driver ask me where my bike was!
Also... exit through the front of the bus, and TELL the bus driver that you're getting your bike off the rack. That way they won't drive off with your bike.
Edit: as far as PERSONAL stories.... When I came back from my first brevet, I was riding the bus and some drunk guy kept asking who's bike that was on the front of the bus... then started saying it looked like HIS bike... when he was getting off the bus, he said he was going to take HIS bike... I didn't say a word, just got up and followed as far as the steps... He looked at the bike, then at me .... then turned around and walked away.
Funniest part was that he was at least 6" shorter than me, and my bike barely fit me... he wouldn't have even been able to pedal it!
