From the New York Times tonight:

Oil prices slumped below $100 a barrel on Monday as the House voted down a $700 billion bailout plan for the financial markets, raising the specter of slower economic growth and depressed demand for petroleum products. The House leadership, however, plans a second attempt to pass the bill later this week.

Crude oil futures fell $10.52 to close at $96.37 a barrel on the New York Mercantile Exchange. After an erratic and confused week, oil prices have lost nearly $25 since last Monday.

Why? Traders don't think people will be paying for the stuff. They won't have the means to do so. That's how bad it might be.