It's an idea that I've been toying with. Problem though. Most if not all internal geared hubs have axle nuts or spacers or whatever that align them in horizontal dopouts to keep the hub oriented properly so that the shifting works all the time and what not. They would have to be modified to work with vertical dropouts. Sheldon says it would work, but I need to look into it. The other thing is that taking the wheel off to change a flat goes from a pretty simple job to a significantly more complicated one. Way more complicated.
I'm still thinking about the idea and I haven't decided yet, but as is pointed out, they don't tend to work well offroad and rumor is that the shock makes them break off expensive parts. The SACHS are supposed to be more reliable that the shimano though, and the rolhoff 14 speed (at $800 for a hub) has been used for off road tandem racers so it's probably possible. You try it first and let me know.