There was an article posted on Macrumors a while ago citing a source that said that Apple had tried it but was not happy with how the finish would wear over time.
I guess if they can get a more durable finish that didn't look bad after being used for a while they could do it. The fact that they're doing a dark anodized aluminum finish on the new iPhone seems promising in this regard.
It would be more feasible on the iMacs since they don't have to endure sweaty palms like a Macbook does.
It just wouldnt work well for the iMac line though. They'd have to intentionally change the colour of the apple logos on them in order for it to work, and I cant see many people wanting a bright white apple logo sitting right underneath the screen.