My feeling is that some of the areas that have a history of established residencies, like Portola Valley, Woodside, and Atherton to name a few, have a much higher resiliency to price drops. We have seen this to be the case as home values in these areas have not dropped as much as in others.
Nevertheless, they cannot hold out forever. My feeling (just opinion) is that these areas will be the ones to see price drops over the next two or so years - unless there is a economic recovery/ substantial increase in a buyer's ability to leverage/afford to purchase these homes. The buyer pool for homes over $2 M has drastically shrunk due to lack of available financing options (a lot of lenders simply won't loan the money on affordable terms) - the spread between conventional loans and jumbo loans is still quite large.
Of course, no one has a crystal ball to tell either way. The only other factor to consider is, if you really want a home in Portola Valley, how long do you wait before making the jump. Family/life situations tend to be a big factor in making a decision to buy a home regardless of what the market's doing. That may explain part of the sales in the area.