I'm curious as well. Though I've never confirmed it, I always suspected that the memory limitation was based on what was currently available on the market when the board was released. Surprisingly, the manuals for both the early and later versions of the board show the memory support as being up to 784MB which suggests that in reality the boards will support 256MB modules - it's only the original spec that seems to have the limitation shown.
Has anyone tried a 256MB module to see what's really supported?