"Chinese investors sold more U.S. Treasury securities in April than any time in at least seven years...China sold a net $5.8 billion of Treasuries, the first drop in holdings since October 2005...the nation held $414 billion of the $4.4 trillion of marketable Treasuries in April, according to today's report."
So China holds a little under 10% of marketable Treasuries...I don't see that as being a worrisome fact. As the Bloomberg report notes, "Chinese officials have said they have no intention of doing anything that would devalue their holdings." In fact, as an owner of so much of the float, the Chinese government would have a difficult time getting rid of a major portion of their holdings without pushing prices down significantly. So really you could look at China as an ally of the US Treasury.
The political implications of this are important to consider; in spite of occasional saber-rattling, China and the US need each other to succeed. China is a nation with no history of democratic rule, and until recently primarily negative experience with Western capitalism. The twentieth century was a period of extreme political and economic instability for China, to say the least. So Chinese leaders and the country's people as a group are learning as they go along in managing a capitalist economy. It seems that ideally China would make a political transition like that of South Korea, which has shifted from dictatorship to an electoral democracy.
From the US perspective, China is at least moving toward a more open economic system; unlike Russia, which seems to be reverting to a kleptocratic oligarchy. Given that the US has developed deep economic ties to China, the US has a strong interest in improving its political relations with China.