What I couild infer from my own testing is that it will most probably depend on
1. Whether you can find RAM that clearly states that it offers a 667 MHz preset (written into EPROM on the RAM) alongside the 800 MHz preset.
2. Whether the presets built into the RAM are too aggressive at 667 MHz.
If you are running RAM where there are no 667 MHz presets, or where the timings at 667 MHz are too aggressive, the T61p won't boot, or will make it part way, and then BSOD on you.
I tried some Kingston Hyper-X 800MHz DDR2 SODIMMs, which were CAS 4, and they supposedly had some standard presets to run at 667MHz. The timings were 3-4-4-10, so I thought -- what the hey? Why not! It didn't fly. Stick with CAS 5, and you might have better luck.
See this thread:
http://forum.thinkpads.com/viewtopic.ph ... it=bananas
But definitely report back if you find that it's not working at all. That'll be a sign either that a BIOS fix is needed, or that the Intel chips don't like it.
What I mean by that is that I tried every conceivable system tool available -- you name it, I tried it -- and the PCBus Controller will NOT read the timings off the EPROM on the RAM chips themselves. (I needed to figure out whether Kingston had actually provided 667 MHz timings for the RAM or not.) The best you can do is with Everest Ultimate, where you can get settings reported indirectly from the Northbridge chip on the motherboard.
See this troubleshooting thread:
http://forum.notebookreview.com/showthread.php?t=332839
That seems to be a BIOS problem, but there's also documentation out there that says that the Intel Northbridge chip "supports 667 MHz" -- i.e., not 800 MHz -- even though it has a 800 MHz Front Side Bus.
It would be great to get some more datapoints!
a.k.a.