i think ppl have said thats more rumor with nvidia then actual truth..
ATI has done this.. thy have used defective high end cards as standard cards, since all of the piplines wernt intact.. if nvidia had doesn the same now with the 6800, chancess are, u do not have 4 extra pipes. maybe 1 or 2. and the risk of u just rendering your card worthless instead of getting more pipes, not realy worth it.
its true that there may be a coupl standard 68's out there with 4 extra pipes, but y would thy go threw the trouble of disabling a card with 16 pipe's just to run one with 12... my guess is, that u will probably get struck by lightning b4 u run a cross a standard 68 with 16 actual pipes that have no defects.