Best answer: probably nothing. If you are not already short, it is too late to get short. If you are long, you may be stopped out of positions today. For some positions experienced traders may want to pull stops for the first 30 minutes of trading and then reinstate them at the opening range low. Don't do this impulsively, but only if you have it planned out and have the discipline to go ahead and reinstate the stop and take the loss if necessary.
Our hearts go out to those affected by the earthquake and possible nuclear reactor failure in Japan. And yet we cannot let that affect our trading decisions. History shows that markets do not typically begin bear markets or extended corrections after such events. Even after 9/11 markets bottomed just a few days later on 9/21/01. After Katrina it was about six weeks later. Chernobyl and Three Mile Island occurred at the beginning and early parts of a major bull market and you probably can't even tell on a chart of the markets when they happened. Even the onset or wars tends to be bullish, so we must put aside our fears and expect that after the initial shock, markets will resume their uptrend.
The best interpretation of the current market action is that it is wave [c] of 4 or (c) of . The most likely support levels are the November and April 2010 highs and the 200demas. At this point the 200dema is a huge target. Wave [c] = 1.618*wave[a] for the SP500 at 1243. But lower support is at 1227, 1220 and 1211. For the Qs, we have support at 54.04, 52.90 and 52.56. These seem like the most likely targets, but will probably not be seen today. Volatility on the other hand may reach its maximum today so some option trades may hit their highest levels today.
Of course the selloff today will have those calling for P3 down saying this is a 3rd wave. The problem is that you cannot tell the difference between a 3rd wave and a C wave and for stocks the 5th wave after a panic usually only retests the 3rd wave low, so logically one should not be counting on that 5th wave. If it happens then you can adjust your plan accordingly because there will most likely be a retracement.