Washington's Urban Institute noted that the 1990's saw a reduction in the incidents of violent crime, while 2005 and 2006 saw a spike. At the same time, iPod sales were soaring. In 2005 Apple sold 42 million iPod, and almost 90 million in 2006.
Now, we all know that iPods are stolen frequently. They're easy to grab, users are frequently "in their own world" while absorbed in their music and they sell well. It's not difficult (unless the thieves are idiots) but that doesn't mean that a single factor - iPod popularity - is responsible for the national violent crime rate.
Use common sense when walking around with your iPod. Pay attention to your surroundings, keep it inside your clothing if possible (a jacket pocket) or even in your hand and, most importantly, ditch the tell-tale white ear buds. Or just encase it in machined aluminum.