ACTUALLY, there is NO moisture between the case..

To  YOUR SURPRISE, I've actually purchase (http://www.shieldzone.com) an INVISIBLEshield to safe guard my laptop's outer shell and the palm rest on the keyboard.

So as for those darker spots, its because the shell is sticking to that invisibleshield, it kinda has a grip to it if you can catch what I mean. 

BTW, the INVISIBLEshield is a GREAT product, I have it on my iPod, my Razr, and my Macbook, it adds great protection and it keeps for minor and major scratches. I would recommend to a newly purchased macbook (mainly white). 

If you notice on the palm rest, its discolored, well thats NOT on my macbook its actualy on the shield itself, so if i peeled it off it would be SPANKING WHITE!!  I got the shield a week after i got the laptop so there is almost NO scratches on my macbook, not to mention i got the red case a little after that so everything is in tact and is like brand new!!

INVISIBLEshield has a LIFETIME warranty so if it gets too dirty, i take it off and send it back and they send me a replacement FREE! =D