Since 2005, the $SPX has "comfortably" traded between a 12 month forward PE of 12-15 times. Below 12 has been an extreme reached in early 2009, mid-2010 and October 2011. Above 15 times marked the top in 2007 and April 2010. Over the last 10 years, the average has been 14.2.
Keep an eye on this. At SPX 1500 and assuming consensus $112 in FY13 EPS, the $SPX is trading at 13.3 times.