Currently china's holdings of usa treasury debt ($1.16 trillion ---> http://www.treas.gov/tic/mfh.txt).
USA cash deficits (issuance of new debt) [approximate to 1/10 $trillion)
fy 2008 $0.5 trillion
fy 2009 $1.4 trillion
fy 2010 $1.2 trillion
fy 2011 $1.6 trillion (projected)
While china does hold a lot of USA treasury debt, they are hardly the only buyer. As of late the USA treasury has been issuing more new debt in one year than china's entire holdings of said debt.
I don't believe the claim "china could bankrupt the US" is strictly factual. As we continue to issue more debt, the impact of any hypothetical liquidation of USA treasury debt by china, would become more and more muted.