From Morgan Stanley:
We expect the USD to remain under pressure today as the market becomes concerned that the Fed may hint at QE3 tonight. If the Fed did surprise tonight, this would initially be a USD negative and we would expect our new stop limit to be hit. We would then look to re-sell EURUSD at higher levels.
We would also expect any further easing undertaken by the Fed to lead to coordinated intervention. This would be necessary in currencies such as JPY and CHF, which would both otherwise rally strongly against the USD, to the detriment of their local economies. Hence, we would expect the US to stand by both Japan and Switzerland to limit appreciation pressures in the event of further easing by the Fed.