You can easily be mugged and have your expensive bike stolen. Some years back some enterprising chaps in Houston were stringing high test fishing line across the bike path at the base of a small decent and stealing the bikes of the fellows they knocked senseless.
The best way to protect your expensive bike is simply NEVER ride it! Get some 17~19mm security chain and an Abloy PL362T Protec2 and secure the bike to a ground anchor in a secure vault with alarms and video.
That is true, but bike mugging is much rarer than bike theft; having said that they had a rash of bike muggings along the Los Angeles River, and the Santa Ana River Trail, but with all the homeless living down there why in the hell would you ride through there alone anyways? Unless you had a gang of cyclists with you, then you would be left alone. Not sure if LA police have stopped that crap along that river or not, I do know that it's still tent city down through there so that would lead me to believe that the cops are pretty much powerless to prevent bike muggings.
They estimate that ALL the bicycles you see in those films have been stolen one way or another, and most that were stolen were sold. I don't live in LA anymore so not sure if the City cleaned up those areas, I heard that they had, but to what degree I don't know. But there are other large cities with similar situations where you take a risk of getting biked mugged.