In the US, for instance, government bonds already were an important item on the balance sheet of the Federal Reserve before the crisis. The Federal Reserve typically implements its monetary policy through purchases and sales of government bonds.
Let me also emphasise that all the non-standard monetary policy measures taken by the ECB are temporary in nature and complementary, rather than supplementary, to our interest rate instrument. At the same time, we should not forget the adverse side-effects of interest rates being kept at very low levels for a long time.