Tom Bihn. http://www.tombihn.com/laptop_bags/TB0104.html
The Brain Bag with a pair of Vertical Brain Cells. The Brain cell is a suspension type sub-pack with corrugated polymer covered with some tough nylon specifically for your laptop/iPad and the smaller ones hold a pistol very nicely. It also deletes the visible profile of a pistol. The Brain bag has several other exterior pouches which hold mags/knife/goodies very nicely and discreetly. It's not a purpose built gun bag, it's a purpose built laptop pack with extra heavy duty construction and works well. It's what I use.
If you're stuck on a messenger type bag, then the ID or the Imago are good options with a Cache bag to hold the pistol. There's no fast way to draw from any of the above but it does keep the pistol handy for those occasions when you have a few seconds to decide to pull it out or not. For me it's mostly a way to keep a pistol on me and be extra discreet about it.