I just called the US Coast Guard boater safety office in Washington DC to verify. There are absolutely no Coast Guard requirements for flotation for strictly sailing craft. The person I talked to, Mr. Ikenberry (sp?), said that some manufactures "elect" to provide boyancy even when not required (mentioned Walker Bay).
The text of the statement is:
SAILBOATS (EXCEPT INBOARD AUXILIARIES), CANOES, KAYAKS,
INFLATABLES, SUBMERSIBLES, RACEBOATS, SURFACE EFFECT VESSELS
AND AMPHIBIOUS VEHICLES ARE EXCEPTED FROM ALL REQUIREMENTS
EXCEPT FOR THE HULL IDENTIFICATION NUMBERS.
Another interesting point: Even with the flotation requirement for power craft, the requirement is that the boat can submerge but must support the weight of the crew or give them something to hang on to:
must support... two-fifteenths (2/15) of the persons capacity and 25 percent of the dead weight. Basic Flotation is designed to give the survivors of a boating accident in which the boat has swamped something on the hull to hang onto...
For a complete description of how these things are measured go to:
Coast Guard standards (for home builds). 
