This is one of those days that will be fun to watch. There's no particular reason for anything to happen one way or the other, so far. Bush is going to give a speech about economic stimulus, but there will be no surprises in it, probably. Anything could happen - even a big move up or down. On a day like this it's all imagination and emotions without connection to reality. It's days like this that keep people in money market accounts.
There will likely be more down days and weeks in the near future, including some more plummeting like yesterday. But by the end of the third quarter, those that kept their heads and dollar cost averaged their way through it will be glad they did.
Things are looking up. The housing/credit problem is ultimately a good thing, Bush has stopped threatening to start another war, and soon there will be a new President. The economy is, and has been, actually pretty good under the covers. The falling dollar and inflation due to energy costs are a risk, but there's plenty of room for that to change (oil is not actually in short supply, in reality, for example). Emerging markets that do not depend on the US are still doing well. The stock markets will get run up in '08 in anticipation of economic growth in '09.