New MacBooks and MacBook Pros Support Up to 6GB
This finding is consistent with the experiences of previous MacBook Pro owners. 8GB of RAM has been shown to cause stability issues under Mac OS X when the additional RAM is accessed. For some reason 6GB appears to be the practical limit on the latest MacBook Pros, despite hardware support for 8GB.
Apple officially advertises that the new laptops can only support up to 4GB of RAM.