The Views from the Top Board (the best board for Northeast info) has a post on skiing Greylock.
Here are the comments:
A ski instuctor from Jiminy Peak skiied it last year, I've been told. I did not witness it. I tried to do some recon on it last summer and it was so dense with rasberries I could not get very far down it. It would neeed 4' of cover to be skiiable in my oppinion. I wouldn't attempt it as rescue would be next to impossible. IMHO this is the most extreme descent below treeline in the east. It is in excess of 45 degrees for a long continious section and doesn't follow the falline very well. 2000' vertical on the way up and the way down
There was an article in Coulior 5 or so years ago that said that they were skied a couple of times in heavy snow years. I imagine that last year would have been such a season...