great source:

Where Are We with Market Valuations?

10/10/2010:
The Stock Market is Fairly valued. Based on historical valuations, it is likely to return 6% a year from this level of valuation. This page is updated daily with the market.

Total Market Cap and US GDP

The Ratio of Total Market Cap to US GDP

The Predicted and the Actual Stock Market Returns

What returns can we expect from the stock market?

As of today, the Total Market Index is at $ 12196.2 billion, which is about 83.5% of the last reported GDP. The US stock market is positioned for an average annualized return of 6%, estimated from the historical valuations of the stock market. This includes the returns from the dividends, currently yielding at 2.1%.

As pointed by Warren Buffett, the percentage of total market cap (TMC) relative to the US GNP is “probably the best single measure of where valuations stand at any given moment.”

More at the source on:

1. Interest rates
2. Long Term Growth of Corporate Profitability
3. Market Valuations
Also....

As of 10/10/2010, the stock market is likely to return 6% a year in the next 8 years.

Note: If this stops updating, look here for Wilshire total market value. (I'd use full cap)

Today it is 95% TMV/ GDP