I suggest that you look for frame that has the rear brake mounted on the middle par of stays instead of on the seat stays. Otherwise you have to use a special "upside down" rear brake like on Neals bike above or end up with some wierd brake cable routing like on my mixte.

http://sports.webshots.com/album/558351622LHIONO

I'm curious about the lug that bionicycle mentioned. Mine has no such lug and I haven't seen on that has.
I've never seen one with the twin tubes welded directly to the seat tube either. There is normally a reinforcing plate there. If the tubes were welded directly to the seat tube, there would be no room between them for fenders and possibly for tires.