"Failure to Deliver" in the US Treasury market

At this post on credulous_prole's utterly correlated blog page Dr. Susanne Trimbath's work on the mechanics of the "failure to deliver" in the US Treasury debt market is explained.
Dr. Brad Setser's analysis of the latest Treasury International Capital (TIC) data for November 2008 shows a marked shift from long term to short term Treasury debt purchases by foreign central banks. His previous analysis shows a large volume of private purchase of US Treasuries in 2008.

Here's an extract from Dr. Susanne Trimbath's article:
"Because the financial regulators do not require that the actual bonds be delivered to the buyer, your broker credits you with an electronic IOU for them, and, eventually, with the interest payments as well. But the so-called “bonds” that you receive as an electronic IOU, called an “entitlement”, are phantoms: there aren’t any bonds delivered by your broker to you, or by the government to your broker, or by anyone. "

I strongly advise the reader to go through her entire analysis.

I think Dr. Trimbath's analysis might have a connection with the reduction in long term Treasury purchases by foreign central banks. I could be wrong here.