After three pages and nobody bringing this up I'm sure I'm wrong, but I thought the Intel chips themselves were restricted to 4GB RAM. I don't know how that works, but that's what I had heard somewhere. Can anybody say whether there could be a limitation in any of the other hardware that would restrict the MBP to 4GB?
What does the FSB have to do with the RAM? FSB is the link between Northbridge and the CPU, it has nothing to do with the amount of RAM the system supports.
IIRC, Santa Rosa and Penryn only support 4GB of RAM. And I'm talking about the mobile chipsets here.
People, there's been a few users on these boards that have their previous model Penryn's running fine w/ 6GB of RAM. In fact they are far snappier that way. Keep in mind its DDR2 memory.