One Source Tactical has partnered with Blade-Tech to design and create the WRS RMR Equipped Tactical Holster. This is the first production level 2 retention holster that I am aware of that is designed to accommodate a handgun (Glocks in this case) with a red dot sight (RDS) mounted on the slide.
The slide mounted RDS concept seems to be gathering more and more momentum now that there are some optics, like the Trijicon RMR, that really lend themselves well to this application. One Source offers their own package of slides that come complete with an RMR and gunsmiths, like Bowie Tactical Concepts, report that they are milling slides to accept RDS at an increased frequency. The introduction of this holster would seem to lend further credence to the growth and acceptance of this concept.