With bikes, at the moment, it seems to be a significant problem: there are a ton of bikes in Copenhagen that end up stuck somewhere because the owner lost the key or forgot the combination. Sometimes the owners get a bolt cutter and cut the lock off; other times the police eventually show up to an area that's getting full of abandoned bikes, cut all the locks (after tagging them for a few weeks, which basically declares open season for anyone else too), and haul out a whole set of bikes. I'm not sure the economics work out for it to be worth doing the equivalent of going back to a car dealer for a new set of keys, when just buying another used bike costs $150-300. A car's worth doing it for because it's expensive, so even a dealer charging you $200 is worth it. But with bikes, even with the current economics many people don't bother: figuring out how to cut a U-Lock is more hassle than buying another cheap bike, and not much cheaper.
