Borescope recommendations (features)
This one has really good reviews on Amazon, for what it's worth.  It comes with 3 attachments: a hook, a mirror, and a magnet, all of which sound like nice things to have.  There are also extension cables available for going longer distances inside the wall (the included cable is 3 feet).  Here's a pic:


The price seems really reasonable at $160 (as of Nov. 2010).  My guess is that if you went to rent one, you'd be limited to more commercial-grade models and that if you wanted to keep it for a few days you'd be looking at something close to that just in rental charges.