Where can I get historical P/E ratios for stock INDICES like the S&P 500?

For some reason, this information is not readily available at research centers in ETrade, Charles Schwab, Y! Finance, etc.
Need to get this up-to-date and preferably daily updates.

bigcharts has the data in chart form:

http://bigcharts.marketwatch.com/javachart/javachart.asp?symb=spx&draw.x=20&draw.y=11

you can select P/E as one of the lower charts – the current PE of the S&P500 is 16.86