I had mine replaced 3 times...
last time the employee told me the issue was the Macbook getting hot & thus easing up the process of trapping oil & dirt from your hands... nothing much they can do about it & I have noticed the plastic looking/feeling the same each time a new part was added... so I guess they didn't change the manufacturing process that much, but it differs from the iBooks wich cleaned much more easy since they never heated up this much and thus not 'burning' the oils and so in the plastic parts
anyway only the first time I got my macbook in for the discoloring, the two other times there were more structural problems with the hardware & they replaced the bodyparts anyway because the looked bad, not because I was telling them to do so 😉