Below are a few excerpts by Morgan Stanleys David Greenlaw. (who also had it figured out earlier than most) The entire thing I can be found here. It is the best description I have read explaining a very complicated topic.
QE2 departs from the textbook. The issue is confusing because all of us who took a basic undergraduate Money & Banking class learned that a central bank's open market purchase of securities was effectively the same thing as printing money.[My comment - no one actually challenges the textbook. No one challenges perceived truth!!!] But the experience of the last few years has taught us that this logic is not always correct. In fact, Fed officials have been reluctant to adopt the QE terminology because the impact of asset purchases is all about rates - not quantities.
Fed will respond to inflation as needed. Interestingly, the market moves that we are seeing in currencies, commodities, inflation expectations, etc., appear to reflect a belief that the Fed has been printing money - or will do so at some point down the road as the money multiplier normalizes. Bernanke tried to address this point in the 60 Minutes interview. He indicated that the Fed could raise rates in "15 minutes" if necessary and that he is "100%" certain of the Fed's ability to respond to an inflation threat. Of course, it remains to be seen whether the Fed will follow through on this pledge - and it remains to be seen what the FOMC will consider to be a legitimate inflation threat. But the market moves that appeared to coincide with the reintroduction of Fed asset purchases reflect speculation - as opposed to a fundamental supply/demand shift - because there hasn't been any money creation to date. Ultimately, the success or failure of the Fed's asset purchase policy will depend on an interest rate transmission mechanism, not a quantity channel.