If you really need this feature, at the exclusion of much else, I think the best way to go is something like ThinkTank's belt system (http://www.thinktankphoto.com/catego...s-pouches.aspx). The trade-offs are pretty obvious- it's not really a camera bag anymore, you will look like a sports photographer, and there is always some risk with a system like this.
I try to do multiple bodies to avoid lens changes, but that isn't always a possibility.
Well I was going to say something similar as most of the time I carry two cameras. But I'm sure the best solution is this:-