That seems to be the most common idea around - some sort of elastic neoprene "pack". Not having tried it, I think it would be hot. The Hill People Gear rig is one that I've heard frequently cited, but it too looks amazingly uncomfortable in the heat - and having something around my chest for a several mile run doesn't seem appealing.
I think I would prefer something like an Amphipod or a Nathan type waist pack. There are plenty of reviews on them, but I can't find much about someone stuffing a pistol into one (a couple references to an LCP, but nothing about a PPS or larger).
Anyone run with a camelbak?
Thanks for all the great feedback so far!