Here's a cool a graphic displaying the share of US Treasury bonds held by different nations with China holding the most US debt.
Although China is now seen to be diversifying its foreign exchange reserves with the recent purchases of European "periphery" bonds (i.e. Portugal and Spain) the country remains heavily invested in US debt. Premier Wen Jiabao had previously called on the US to ensure the safety of its bonds as future increases in the US bond yields, caused by rising deficits which will require more funding, would devalue Chinese holdings*.
However, despite a small drop at the end of 2010, it would not be in China's interests to abruptly and dramatically move away from US debt. This would lead to further deflation of the dollar and a fall in American consumers' demand for Chinese goods.
What is most likely to happen in the near future is that China will continue to slowly let the yuan apprecate in a controlled manner while also diversifying its sovereign debt holdings.
*Explanation of this here