Coming off a story about a Wal-Mart employee intercepting conversations and text messages between Wal-Mart PR and a NYT reporter, Slate takes it upon itself to teach you how to do just that with your own dinky cell phone.
First on the list, predictably, is copying the interceptee's SIM card, sending all of their phone calls (if you're on the same tower) and texts straight to you. The second method uses a (very illegal) firmware update to turn your phone into a radio, picking up "all the texts broadcast on a given channel," though obviously you'd need to know the network and be close to the base station for it to work. While less precise, it's also cheaper than buying the equipment to clone your mark's phone.
Lastly, you could get ahold of some actual Bond-style toys used by the DHS and other three-lettered agencies, but that sounds like a ticket straight to either the front page of the NYT or the bottom of some classified prison in a country whose name we can't pronounce. Not that the first two methods are any less illegal, so get your Linda Tripp on with caution.