The amount of margin debt as reported by NYSE has plunged to $233 billion in October (latest figures available) from a high of $381 billion reached in July 2007. It's pretty certain that it has declined even further in the last 1 1/2 months, given the recent performance of markets.
I must caution that we live in the Age of Derivatives and therefore speculative leverage can be taken on in many more ways than plain old margin debt (e.g. CDS). Still, there is no question that de-leveraging is happening very fast.