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I always think numbers like these are pretty meaningless, since they don't necessarily take into account all of the right variables. While I see a lot of postings for stolen bikes in Portland, 90% of those are either unlocked or cable locked bikes, 9% are house/garage/apartment break-in thefts, and the other 1% are properly U-locked bikes in a public place that were sawed or jacked apart.
Great cities for cycling, like Portland and Eugene, probably tend to have higher theft rates because you get many more less-experience cyclists who use junky locks or use their locks improperly. Just like higher numbers of cyclist accidents, it can indicate that you have a lot more people doing the activity and not that it's more dangerous.
P. Squiddy - you are absolutely right - more bikes = more theft by virtue that there are more bikes to steal. This is why theft numbers are so high on college campuses, too. There are a ton of bikes on campuses around the world. Thieves know this and target those areas.
You are also right that a lot of bikes are not locked or not locked properly. That's the whole point of doing the list - to draw some attention to what can happen if you don't lock and lock properly. Anything that gets people thinking about securing their bike is a good thing. We want everyone to keep what is theirs and just piss off thieves all over the place.
Fritz - thanks for helping to pass the word about properly locking!