OWC has confirmed that the 8GB modules do work in the 4/6-core 2010 Mac Pro! According to OWC, using 3 modules shows a ~ 15% memory bandwidth gain over 4 modules, so the configurations with a * at right ar the optimal ones. Whether real-world tasks are affected by this small difference remains to be tested, but in past testing I never measured more then 3% hit from using 4 modules instead of 3.
With this confirmation, my concerns about the 4/6-core 2010 Mac Pro are laid to rest. My top pick remains the 3.33GHz hexacore model, which is not to say that the 2.8GHz or 3.2GHz models are not also excellence choices, and bound to satisfy virtually every user.
See the Buying Tips page for the Mac Pro.
OWC sent me some screen shots with memroy installed in the 2010 Mac Pro quad-core: