NPR had a very good Planet Money segment on the 7th dealing with bike theft. http://www.npr.org/blogs/money/2012/09/07/160753071/the-economics-of-stealing-bikes A good listen. I breaks down to the fact that bikes are easy to exchange for money, goods and services at little risk of getting caught. And even if caught, there are very few penalties applied.
Long/short of it: LOCK YER' BIKES UP!