I wanted to just throw a quick point into this conversation. Returns mentioned above are average returns for a 20-year period. The problem with using average returns is that they mask when the risk shows up. What's important is when you take the money out of the stock market, what is the market doing. If you have a big drop just before you need the money, stocks can hurt you, even if the average return was good for the whole period.
Of course, this situation can be mitigated by slowly ramping down from stocks and by holding a diversified portfolio. Even having said that, I think most people don't understand the risk involved in stocks.