Freewheel, Horizontal Rear-facing Drop-outs and Rear Disc Brake

BillB sliding dropouts are the way forward IMHO if you want discs and single speed:

http://www.paragonmachineworks.com/

My Voodoo mountain bike has a very similar setup for running a geared hub.

or an EBB can be very effective too.

was talking about this in the bike porn thread, thanks for reminding me what they are.