Apple officially describes the foam pad "part" needed to fix the problem as the "RAM Gasket Fulfillment Kit." Again, "the kit is available at no charge from Apple-authorized service providers. Ask for part number number 922-3103 (GASKET,FOAM,KEYBOARD). Or you can call the Apple Assistance Center and request the Kit. (Soon there will be a phone prompt in place to route you directly to a team supporting this fulfillment.)"
Update: Edward Robinson writes that the RAM Gasket kit has also been used for similar problems in the Powerbook 190s and 5300s.