This is becoming more common. You still need to slope the floor so the easiest way to do that is to recess the floor where the shower will be so you can have the slope to the drain, but keep the edge even with the floor. Recessing the floor can be a pain, but it may be necessary since you'd not want any significant lip at the threshold into the room, either, although you could put a small ramp there. You will need to waterproof most, if not all of the floor in the room. Check out Kerdi at www.schluter.com for one method that works well. The room will be kind of small to allow a wheelchair to get in there and turn around. A bench in the shower would be a nice touch.
Go over to www.johnbridge.com and look through their library (liberry (sic)) for some pictures and other advice. Once you've looked that over, post your questions.