Hearing the Quark digger mentioned in the mapping help thread reminded me of the hollow maker. It's an even more "extreme" version of the digger: it provides the outer bounding box brushes implicitly (and only digs against them, so you can put other brushes in the room with no problems). Basically you can map with one brush per room, and intersecting brushes in the hollow maker automatically become corridors.
I know this sounds like a recipe for disaster, but I was thinking that this could be an awesome tool for building a throwaway map prototype. It could at least improve your "sketching" speed, and get a map to a point where you can work on gameplay really quickly.
I haven't really tried doing a map this way.. so maybe it encourages box map design, but I'm planning to try it out on my next map.
http://quark.sourceforge.net/infobase/maped.tutorial.advanced.html#wallhollowmaker