First, the MB must be a Core2Duo. CoreDuos need not apply.
We have tried a 2 Gb module in a BlackBook, and it works fine. Recognizes 3 Gb in About this Mac and 2 + 1 in System Profiler. We did not benchmark performance 2 Gb matched vs 3 Gb unmatched however.
I cannot guarantee this will be the case for every brand of 2 Gb SODIMM, of course.
Luckily I do have a C2D BlackBook, and if I understand correctly, it will do 3 GB of RAM? But what will that do to my pathetic little GMA 950? Oh and what if I put in 2, 2 GB sticks, would I get the 3.2 GB the MBP does?