The month of Jan 2008 has shown to me that the Dow can drop 600 to 1200 points faster than Carl Lewis. ( 100 m sprinter LA games). Electronic trading and fear combined with the mass of retail online trading a Oct 1987 crash still is a very real physically possibility. Sure there are limit down pauses, but for the DOW to gown 3000 to 5000 points in a week or two is very possible. Also with 45 trillion dollars of futures contracts out there, there is plenty of positions to motivate such a event. I guess the signal for this to happen is a very clear ASSET DEFLATION event. What would this be, unemployment jump, employment insurance claims jump, house price falling, interest rates jump. So 1987 can be repeated...and I dont think the PPT would be able to stop it.