Mentioned here by Peter Coy:
The abrupt shrinkage of the budget deficit this year caused the Treasury Department to cut back on issuance of debt in the shorter maturities that are sought after on Wall Street. Foreign demand has been strong, partly because some European government securities that had been used as collateral are no longer considered safe enough. Finally, the Federal Reserve, in its efforts to stimulate the economy, has been soaking up $85 billion a month of Treasuries and mortgage-backed securities. Some market participants argue that the Fed is planning to taper its bond purchases in part to relieve the collateral scarcity.
