Need to be undercover? Well, make yourself impossible to photograph.
Get some infrared LEDs. They're undetectable to the human eye, but that's not the case with cameras. Wire them to the brim of your hat and you've got instant invisibility to any camera -- paparazzi, Big Brother or otherwise.
This works great on the brim for a snapshot-phobic face, like in this video by hack pro Minmak. If you're not a hat guy, try embedding the LEDs in sunglasses. Maybe a sweatband.
For informational purposes only (illegal): This same technology has been used to block license plate numbers from traffic cameras.