"In essence, the Fed now will print money to buy as much as $900 billion in U.S. government bonds through Juneāan amount roughly equal to the government’s total projected borrowing needs over that period."
—The WSJ wrote it better than I did (but I wrote it first).
The FT added a bit more:
“With the US Treasury expected to issue $1,200bn in debt over the next year, the Fed’s purchases under QE2 are expected to absorb much of the new supply and keep yields low.”