The Timbuk2 Snoop Camera Messenger (Small, Black/Gunmetal) would make an excellent spy. On the outside it plays it cool with dark coloring and classic messenger styling, but on the inside it's calculated, cautious and a little bit sneaky.
The fully padded, totally modular interior is designed to house camera lenses, bodies and batteries. Simply move the walls around to fit the cameras you carry. If you really mean business, you can secure your tripod with the adjustable straps on the bottom of the bag. And when you're not packing (cameras), simply remove the padded insert and rock it like a classic messenger.