Recently I bought a 3DS and have started playing around with it. They encourage you to leave it on and carry it around with you. As a computer software developer and reader of news about bot nets, device hacks, and surveillance this got me to thinking about the implications of this. Many people likely take these devices to work, near potentially sensitive networks and systems, and near open wifi. If someone were to hijack these devices via that 'streetpass' system they could spread a bot net far and wide fairly quickly and most people would never even realize it. They could be commanded to initiate denial attacks, rip through unprotected systems, plant backdoors, track individuals, and do any number of other undesirable things. Entities we may trust could be doing this to us right now.
This then got me to thinking about how easy it would be track someone with common technology available to any person on the street. It wouldn't take much; think about any item that you regularly carry around with you. Your coat. Your shoes. Your pants. Your purse or backpack. Your wallet. Any of these items could have a tracking device inserted into them and most people don't have the capability to detect them. Devices that could provide an ping signal aren't that expensive. The real cost is in attaching them to the population at large, but infiltrating and tracking a single individual or group of people would be trivial.
I guess this goes to say that if someone wants to track you then they probably already at. The broad tracking of 'meta' data, phone calls, and cell phone locations is just their icing on the cake to hold everyone hostage at once instead of just specifically tailored targets.