This isn't right. China holds between 22-25% of US debt, which is a plurality, not close to a majority (http://www.treas.gov/tic/mfh.txt).
China has also indicated that it would prefer to buy less US debt, though it's unclear if this is posturing or not. Both countries are over each other's barrels, to mangle the metaphor. If you owe the bank $10,000 and you walk in and tell them you're struggling, they'll just look at you funny. If you owe the bank $10,000,000,000 dollars and you walk in and tell them you're struggling, you better believe they'll want to work with you. The USD makes up about 65% of foreign reserves (down from about a steady 70-ish% between 2001-2007). China doesn't keep buying USDs for fun. They buy them for profit, stability, and for preventing the appreciation of the yuan.
They hold a majority of our government's debt...