Sorry crvm....have to contradict you on the model I am familiar with.
The 737-800 autopilot will disconnect at 158 feet rad alt with single autopilot engaged (certified 'minimum use height' by boeing) and will also disconnect with both autopilots engaged but failure to recieve flare arm annunciation at approximatly 350 ft rad alt. this would occur only if the was an FCC problem...not a normal scenario obviously.
As long as you don not get flare armed the aircraft cannot autoland.
Ref boeing ops manual ch4 page set 20