The S&P 500 is the white line in the chart using the right scale, and the US dollar is the orange line using the left scale. Dollar rallies (shown as green trend lines) and S&P 500 sell-offs (shown as violet trend lines) occur coincidently in time. Also, dollar sell-offs (shown as red trend lines) and S&P 500 rallies (shown as blue trend lines) exhibit this same relationship.
However, the last few days have created an interesting divergence in this relationship, as shown in the boxes at the far right of the chart. The S&P 500 has been falling but so has the US dollar. A few days do not make a trend (or in this case a relationship change) and we would expect the dollar to rally if the selling pressure continues. If this relationship does change, it will be very telling that the market no longer views the dollar as a safe haven play. I wonder if the Federal Reserve would see this as a sign to reassess the effect that Quantitative Easing has on our currency.