According to an article on cnet Snow Leopard boots by default into a 32-bit kernel and mentions that macbooks with an older, 32-bit EFI chipset (which handles the early boot process) cannot boot the 64-bit version.
Although having a 64-bit kernel would be good, I'm more interested in the side effect of the 32-bit chipset memory limitation. The same article mentions that a 32-bit EFI chipset cannot address more than 3Gb of ram. Since most likely this is the chipset installed on my mac, I think that's the reason why 4 Gb are not supported. I wonder if the chipset could be replaced. Doubt it, since it must be soldered into the board.
Sadly, that's pretty much the largest reason I'm envious of the new macs. 2 gb isn't much, specially if you're like me and have tons of apps loaded all the time.