depends...
there are now 2GB SO-Dimms out on the market, and the chipset in the Intels do support 4GB (despite Apple's max of 2GB notes)... someone needs to try and let the reporting sites know.
The Pismo's and the Wallstreets each were reported to support 512 and 256 (respectively) officially by Apple, however their chipsets allow double that amount... ie 1GB in the Pismo and 512 in the Wallstreet.