Recently following two sizeable LTROs the ECB balance sheet has grown significantly to eclipse the Fed balance sheet (in nominal fiat/local currency terms). The balance sheet would likely expand even further on new outright purchases. This implies that there could be further downside for the Euro, unless Bernanke acts to counteract.
The ECB's newfound willingness to expand its balance sheet introduces an interesting game theory aspect to US monetary policy that wasn't as prominent in 2009/10. If Bernanke wants a weak dollar relative to other global currencies, he may be forced to act further. Otherwise we could be looking at a much stronger dollar along with the anti-correlated equity prices that come with that.
Of course, who cares if the numerical value of the S&P falls if the dollars that measure its value are worth more? Depending on his actions, Bernanke might.