At GDC Europe 2010 I did a talk together with Chris Schmitz from Ubisoft about free to play monetization. An editor from Gamasutra (Brandon Sheffield) wrote an article about it which spawned a heated discussion about ethics in free to play games (see comments underneath article).
So for your enjoyment read this first:
GDC 2010 talk by Teut Weidemann - with comments
and then their really well done fllow up:
Playing with fire: Ethics and Game Design