Something happening in Japan (with carry trades) or in China (bubble market, I am profiling China currently as I think this is where trouble will start http://lauristonletter.blogspot.com/). I think it will be commodity related or carry trade (hedge funds). China and also I understand Vietnam is up something like 40%+ just in 2007 and about 350%+ in 2006. Vietnam may be ok, but China has completely gone parabolic, just read the articles.
Face facts people, the only way to go is up. 6 months ago, the street was worried about inflation and oil prices, then it turned to worries about slower growth, then it was subprime, and now its bond yields. Stocks continued to climb anyway so the doomsday soothsayers would have either been short squeezed or missed out on opportunities to make money. Buying dips is the only way to go because the reactions have been shallow, its simply not worth the risk to go short right now.
a small 'dirty bomb' explosion would probably lend to speculation of additional events with little chance of stopping them, imho
I see that no one has given the "obvious" correct answer. Thorn calling 1440.81 a market bottom and that we will see 1495.89 within three weeks or he will leave ET.