The last time this subject came up (and just now, too) I looked at Crucial's website and found that they've apparently undergone a change in policy and no longer sell RAM upgrades that surpass Apple's specs.
If I had to toss out a flying guess, I'd wager that was due to pressure from Apple. Apple buys memory from Crucial. They probably got told "knock that off or we will change suppliers".
I work for the Department of Redundancy Department
