This company offers 6GB kits (4+2) for the unibody macbook. Price is quite high though.
http://www.ramjet.com/ItemDescription.asp?Item=MBAL6G
That company also offers 8GB kits for the macbook too. Although Im left wondering if there is something "special" about this ram, that may make it more stable. I assume that is not the case, but I dont know why they would be selling it than.