I admit that I liked holster mode on the Tour but I'm curious how the Droid would support it. AFIK, the Tour detected it was in the holster via the magnet in the holster. The Droid uses that to tell if it's in the dock. As an experiment, I put my Droid in a holster with a magnet and it immediately went into dock mode.
Not saying it's not possible (and it'd be cool if they did it) just curious how they could overload the function of the magnet to multiple uses.
One possibility would be to make the action the Droid takes when it detects the magnet configurable so that you could choose whether it goes into holster mode, dock mode, etc. Of course this isn't a good solution for those of us who have all the accessories since we'd have to change it all the time but I guess it'd be better than nothing since you could at least choose the one you wanted based on what you do most of the time.
The car dock and multimedia dock seem to work by detecting a magnet in different locations on the back of the Droid. Maybe a developer could make one that goes into holster mode if magnets are detected in both locations at once. That way car, multimedia dock, and holster modes would all work without interfering with each other. It's just speculation on my part and it would of course require some cooperation between a developer and a holster manufacturer.
I like the double magnet idea. Although it would still require the case manufacturers to make holsters designed like this. I could see this happening much easier if this was built into the phone rather than an app.