I was kind of wondering the same thing. I know that there are several AR-15 pistols out there on the market. I haven't seen a holster for any of them yet, but hey, I might get one if I could find it.
About three years ago I was in Washington State and was driving down Interstate 5 through Olympia, so I decided to stop off at Olympic Arms and talk to them about my two Olys and how much I like them. I saw and held an OA-98 and talked to the salesman about it, and he told me about an Army chaplain who carried one in a custom thigh rig in Iraq. I'd love to see a picture of that!