use 6GB RAM in portable Macs

According to this MacRumors forum thread you can use up to 6GB of RAM in MacBooks, that support 4GB of RAM.
Update: more infos about this at TidBits.
Update: it seems, that most Macs, that support 4GB of RAM should be able to use 6GB, and the new MacBook Pros 15" and 17" now officially support even 8GB of RAM. More on this soon.