With the Citi Economic Surprise Index (CESI) going negative in late January, there is reason to be concerned about the forward PE valuation on $SPX.
The chart below is from February 2012 (last year). You can see that over the prior 5 years, there was a high correlation between CESI and valuation; when economic measures exceed expectations, multiples expand, and vice versa. This is what you would expect. (In this case, by the way, the $SPX rose 5.5% over the next 2 months; the blue line caught up with the red one).
This chart is from the excellent site by Ed Yardeni. I recommend bookmarking it.
The issue today is that CESI is now below zero while the 12-month forward PE on $SPX is 14.5 to 15 (1520 on FY13 EPS of $100-105). This valuation at the top of the range (chart below is from January 24, 2013).
Something is likely going to give: either CESI will rise quickly or $SPX should correct by several percent in the next few weeks.