There are 216000BTC mined every 30 days (assuming steady six per hour rate), and the volume at Mt.Gox for the last 30 days is roughly 1000000BTC. Now even when assuming that every miner sells every bitcoin they get (which they certainly do not), they would represent only 20% of the market, so it doesn't matter THAT much what miners think about what bitcoin value should be.

I believe that increasingly large portion of the sellers are early adopters with significant amounts of bitcoins, who feel that with current price they can start making nontrivial amounts of additional income by slowly selling, without making a huge dent in their bitcoin holdings. Of course, I don't have anything to back up this claim, other than "that's what I would do", if I had a lot of bitcoins When you wake up one morning, and realize that your previously almost worthless bitcoins you mined "just for fun", represent 99% of you wealth, you would want to diversify, or use that money to buy something nice, even if you expect bitcoin value to rise more in the future. This will serve to stabilize prices (I guess) and spread bitcoins to a larger audience. Maybe that's what we are seeing now in the markets? A lot more sellers than buyers..