The 12" powerbook has 256MB soldered on the mother board and a 256MB chip in the user replaceable slot. If you want to upgrade the ram you have to remove the 256 and replace it with up to 1GB. The 15" and 17" powerbooks have 2 slots and nothing soldered to the mother board so they can take up to 2GB of memory.
I wouldn't expect the ibook to get 512MB on the mother board until the 12" power book does.
I wouldn't expect the ibook to get 512MB on the mother board until the 12" power book does.