The outlined plan of QE3 states that the Fed will purchase at least $40 billion worth of mortgage-backed-securities each month until unemployment numbers are satisfactory. It is interesting that they have not defined what a satisfactory level of unemployment is, but based on historical statements released by the Fed, they are likely looking for levels somewhere between 5% and 7%. Many are calling this QE program, QEternity due to the lack of end date and limits on the program.
Open-ended money printing could be considered positive from the perspective of stock markets, since the message delivered by the Fed is clear that they will do whatever necessary to prop markets up, and continue the illusion of recovery. Interestingly enough however, markets to this point have actually seen declines since the announcement.
Major US indices are down between 6 and 8% in the two months following the announcement, despite prior consensus that prices would rise like they did following QE1 and QE2. The effect of QE programs are losing their marginal effectiveness, and require larger injections, only to see lower marginal returns each time.