4GB RAM on a T60?

Dear Lazyweb,

at work we have bought some of those shiny new ThinkPad T60s with 64Bit. Some of them have 4GB RAM installed, but unfortunately those with 4GB only seem to have 3GB available under Debian/Etch/AMD64.

Since those laptops are running on AMD64 kernels, they should work with > 3GB RAM out of the box, right? Unfortunately they don't and thinkwiki says:

Note: While you can install 4GB of memory, the chipset in the T60 supports a maximum of 3GB usable memory.

while the BIOS correctly shows the full amount of installed RAM!

So my question is: Has anyone out there a T60 running successfully with 4GB RAM or is it really a chipset constraint?