This is a wonderful video of a talk that blogger Greta Christina recently delivered at a Skepticon convention concerning the accusation often leveled at atheists that we are angry. It is an absolutely excellent response. She lists the many good reasons atheists have for being angry, points out that these things bother us because they should bother any decent person, and points out that the "angry" accusation is often just an attempt to get us to shut up. The one thing I don't think she mentioned, but which must be kept in mind, is that the accusation is often just shorthand for "you don't believe in god because you are mad at him." Until the believer has the nerve to make this accusation explicit, however, simply respond to the one he has actually made. Greta's talk should give you lots of ammunition.