Normally the 10 or 13 day moving averages would have little significance as support levels, but since late August 2010 they have been the de facto support lines for the entire move, as the market has had incredible strength. I'd normally look at a longer term MA such as the 20 day or the 50 day as support in a more normal acting market. Even in the past 2 days you can see the 13 day moving average (green line) held as support - it was broken briefly late in the day yesterday but a buyer came in to save the day in a late day surge.
This morning we have had the rare as a dodo bird premarket gap down, and in the early going the 13 day has been broken. As always the close is more important than the intraday action but with this level being such a stringent support it is something to be take notice of. The 20 day (exponential) moving average is 1319 which again is a more normal support area. Much more important in a traditional sense is the 50 day down at 1307. Or if you use simple moving averages, we're talking 1315 or so, which thus far are the lows of the day.
(50 day simple moving average)
After the vicious rally from mid March to end of March, the market has been weak during normal market hours and simply staging gap ups in premarket, and then fading during the actual trading day.
Bigger picture, if things don't change from current course we have what looks to be a double top (bearish) in the S&P 500. I said a few weeks back I did not expect the market to bottom on a gap up and that we should retest the lows - in the near term that was very wrong; but if we can get a break of both 50 day moving averages, it might be a correct thesis eventually.