Joan is right: "don't blame the agents"--at least most of them.
The market value of a property is whatever buyers are willing to pay for it--no more and no less. We're in a buyer's market right now; buyers are speaking with their wallets: they're demanding lower prices. Also, in some markets, the rising numbers of foreclosures has increased the supply of houses on the market. That increase in inventory also is helping to drive the median sales price lower.
Unfortunately, there are many sellers who are still in denial; they haven't accepted the reality of todays market conditions. So, they'll list their properties for whatever they think the properties are worth--often against the advice of their realtor. Their properties will sit on the market for awhile until the sellers soften their positions, and come back to reality. Afterwards, they'll listen to their agents, lower their asking price, and eventually sell their properties at the market rate at that time.