The folks over at Nordic Hardware have scooped up a bunch more interesting information in relation to the behaviour of Nehalem's memory system and how DDR3 will perform on it under overclocked conditions.
As has been mentioned previously, the Nehalem platform has been said to only carry official support for DDR3-800 and DDR3-1066, though some sources are indicating that DDR3-1333 memory support will also be included. Nordic explain that although there has been some confusion as to why Intel Core i7 would not support DDR3 memory clocked at higher frequencies, the first thing you need to be aware of is that it is not the frequency of the memory that is the culprit.
The problem is rather that the memory and the processor internals are fed synchronized voltages. Previous reports have stated that anything above 1.65V would fry a Nehalem processor and it would certainly be hard to get any of today's DDR3 memory modules to any kind of decent speeds at this voltage. But, these stories are exaggerated, but the fact remains that Nehalem processors are not going to like voltages above 1.7V.
So, it would appear the limitation comes from not being able to make use of higher voltages without potentially overheating/killing the processor. For this reason, the memory will not be able to be pushed as much as on a current-day Core 2 platform where the memory controller is seperate, thereby having its own voltage circuit.
Hopefully as DDR3 further matures, we will see modules hit the market with extreme speeds at low voltages to counter-act this problem.