I have a new bag insert from Mountainsmith, and I love it. It's called the Kit Cube, and
here is a link.
The Kit Cube holds a 70-200/2.8 lens with the hood,
and a 40-D with a 17-55 attached. Or it holds two lenses and a flash. Etc. It fits inside my day pack, where it can protect my extra gear on a hike.
Even better, it fits inside my old Timbuk2 *small* messenger bag, which becomes a great travel bag, able to hold camera and lenses, plus all the usual travel stuff. And it doesn't look like a camera bag.
On the subject of the Urban Disguise 40 -- that's my usual camera bag at work. Nice bag. Can hold three big pro lenses and their hoods, and two pro bodies, and various accessories. Haven't tried the backpack strap adapter, so can't comment, but the bag itself is great. Though I might replace it with the Retrospective bag mentioned above.