It looks like Timbuk2 isn't the only one with an insert that transforms a courier bag into a camera bag. The new Tenba Discovery Messenger is a $110 messenger bag able to comfortably fit a 13-inch laptop into its slightly more than foot-long interior. The rest of the space can dynamically swap between holding your camera gear and holding absolutely anything else thanks to a padded camera insert. When it's locked in place, your bag has space for a DSLR, 2-3 lenses, flash, accessories, cables and anything else you can squeeze into it.
However, just pull out the camera insert, and the bag can now be used for all the purposes you can think of using a messenger bag for. It has internal space of 12.5" x 9.25" x 4-5.5", depending on how full it is, and weighs 2lbs empty.
It has a water-repellent nylon exterior and a waterproof bottom panel to protect your expensive gear in case you accidentally put it down somewhere wet — and it comes with an extra rain-cover in case you have to go shooting in less than ideal conditions.
The Tenba Discovery Messenger is available immediately through their online store.