I've been wasting time poking around the Intel data sheets for the 5500 and 3500 series Xeon to find out what the memory limitations really are. This is what I've found (I apologise if it is already common knowledge.)
The 3500 series is limited to 1066 memory and to unbuffered memory and to
a maximum of 24GB with a maximum of 2 dimms per channel. This means
that for the Apple quads 4GB dimms is the limit (16GB). It also explains
why Apple has specified the memory as unbuffered and 1066.
The 5500 series will take unbuffered or registered but not both. This
should mean that the Apple octos could take the (at present) much
cheaper registered 4GB dimms but unfortunately no one has reported
trying this out yet. (I can't see why it shouldn't work as the
memory is connected directly to the processor - I can't see how
Apple could change this or why they should want to.) The memory
limit for the 5500 series is much higher so in future I guess 8GB or
16GB dimms could be used (Samsung have already reported 16GB
dimms) so for future proofing the octo models have a more
flexible memory controller.
The 2.66 and 2.93GHz octos should take the faster 1333 memory
provided there is only one dimm per channel (ie a total of 6 dimms).
In summary
Apple has specified memory that will run on either quads or
octos presumably to avoid confusing customers and complicated the
supply/servicing etc.
Though quads have been shown to go to 16GB this may be their limit
while the octo models may be able to go much higher.
It is probable but not yet been demonstrated that the 2.66 and 2.93 octos could take 6x4GB registered 1333 memory.
The 3500 series is limited to 1066 memory and to unbuffered memory and to
a maximum of 24GB with a maximum of 2 dimms per channel. This means
that for the Apple quads 4GB dimms is the limit (16GB). It also explains
why Apple has specified the memory as unbuffered and 1066.
The 5500 series will take unbuffered or registered but not both. This
should mean that the Apple octos could take the (at present) much
cheaper registered 4GB dimms but unfortunately no one has reported
trying this out yet. (I can't see why it shouldn't work as the
memory is connected directly to the processor - I can't see how
Apple could change this or why they should want to.) The memory
limit for the 5500 series is much higher so in future I guess 8GB or
16GB dimms could be used (Samsung have already reported 16GB
dimms) so for future proofing the octo models have a more
flexible memory controller.
The 2.66 and 2.93GHz octos should take the faster 1333 memory
provided there is only one dimm per channel (ie a total of 6 dimms).
In summary
Apple has specified memory that will run on either quads or
octos presumably to avoid confusing customers and complicated the
supply/servicing etc.
Though quads have been shown to go to 16GB this may be their limit
while the octo models may be able to go much higher.
It is probable but not yet been demonstrated that the 2.66 and 2.93 octos could take 6x4GB registered 1333 memory.