I've experienced the 'no fixed abode' problem twice (both prior to 9/11) - once in transit through LAX (NAN-LHR) and once on holiday in the US when the first night was on the Capitol Limited. In both cases the immigration staff were a little bemused, but with good will on both sides, I had no real problem getting through.
I wonder what happens when a plane-load of cruise-liner pax arrives in Florida? To be fair, I guess immigration there are used to this problem, whereas my cases were (slightly) unusual.