To answer pie's question - yes, the whole of a five-storey building needs to be designed against progressive collapse to comply with British Building Regs. This means that if you were thinking of loadbearing masonry carrying ordinary prestressed concrete planks, that probably won't do. You would need continuity in the floor plate to bridge a missing wall (by catenery or diaphragm action), or some complicated system of secondary beams and posts to do a similar job. Much better to have a framed structure
On Carl's wider point, where should we be drawing the line? Most people would agree there's no need to consider progressive collapse for a two or three storey domestic building, and at the moment British Regs require it for five or more storeys. Progressive collapse got a lot of press in the wake of September 11, but I'm not convinced it was a real issue. After all, the towers DID stand up for quite a while after the attack.
Richard