Max 3 * 16GB quad rank per socket?
"peters out at three quad-rank 16 GB RDIMMs per processor socket, or 48 GB."
As I understand it it maxes out at 8 ranks per channel meaning 6 * 16GB RDIMMS per processor socket, which is 96GB. This is backed up by the HP DDR3 memory configurator which lets you put 96GB in a single processor DL380 G6.
I haven't enough money to buy the DIMMs to test it though.