China is by far the biggest holder of US Government debt - $891.6 billion at December 2010, according to the US Treasury. This is about the same as a year before, but ignores possible purchases via intermediary nations. In 2009, ex-Roubini associate and now NEC adviser Brad Setser plausibly argued that much of the British buying was on behalf of the Chinese.
This is all the more credible because of the UK government's own deep and long-standing financial troubles: Why would one near-bankrupt lend to another? In December 2010 the ostensible UK holding was $541.3 billion - triple the figure from 12 months earlier. Setser's January 2009 estimate was that taking US Treasuries and Agencies together, China controlled $1.425 trillion-worth.
The UK has since increased "its" stake in Treasuries by over $360 billion, though China appears to have been reducing its exposure to Agencies for some time, according to a July 2009 report from the Congressional Research Service:
Data from the Department of Treasury indicate that in recent months China has sought to reduce its holdings of LT U.S. agency debt, while increasing its holdings of short-term U.S. Treasury securities.
This shift from Agencies to Treasuries, and from long- to shorter-date debt, is itself a subtly troubling trend.