The operating earnings for the calendar year 2005 are estimated at $73.77 at this writing. The historical average P/E for S&P 500 is about 15. The "fair value" of the S&P 500 index at the end of 2005 should be around 1107 (73.77 * 15 = 1106.55) with this valuation criteria. Today, the S&P 500 index stands at 1195.98. Assuming from now to end of this year the index goes nowhere, the index is about 8% overvalued. Mind you, this is based on operating earnings, which does not provide the true glimpse of the profit picture.
As Reported Earning estimate stand at $66.13 for 2005. This indicates the "fair value" at the end of 2005 at 992 (66.13 * 15 = 991.95), which would make the index about 20% overvalued.
The "Core Earnings" estimates are not provided by the Standard and Poor's at this time. The common wisdom is that the core earnings will come below the operating and reported numbers, and thus would make even stronger argument of index being overvalued as of today.
Anyway you look at it, the S&P 500 index looks at least 8% to as much as 25% overvalued.