Here's another one that gets tossed around a bunch and some times people do not have a good answer for it or argument against it.
"Well, I have FAITH that god exists and that's all anyone needs. Why cant you have FAITH?"
I added to it a little but it's the same idea. As an atheist you will find yourself being villainized for not having FAITH when all of their other points have failed you will be thrown this curve ball.
Ask yourself, what is faith? Does it make sense in this context?
Faith, is this context, usually involves just suspending ones disbelief and accepting without criticism whatever beliefs the theist is promoting. In addition to Nelson's excellent points about how neither faith nor belief can just be 'switched on', I also like to respond that people in other religions (Islam, Judaism, Hindu, Christianity, whichever the theist is not) also claim that just having faith is all that is needed to see the truth of their beliefs, so why don't they try just having faith in that religion?
Bah, faith. At best it's unnecessary (if you have a real reason to believe something), and at worst it's detrimental to critical thinking (if you don't have a reason, or the reasons you find don't match with the faith, and you choose to stick with the faith anyway).
Faith is pretty much a conversation stopper if you're trying to have a rational discussion. I can't think of any context in which bringing faith into a conversation could be helpful in any sense...
I generally ask them to define faith, then continue like this:
I didn't know having faith was a decision. You just told me it was only a feeling, an inborn knowledge.
If it's rational decision, I've given you my arguments against religion.
If it is an inborn knowledge, I was born without it. That would mean that it was a mistake on god's part. A flawed god can't exist, so it's either me or him in this universe. Can't be both. Who's standing in front of you?
This argument covers two topics for me. The first is abandonment of logic. The second is the personal god and internal feeling of god (evidence not required).
1. Abandonment of logic.
Faith is the antithesis of logic. Once someone decides logic is unnecessary then the floodgates are open. Any argument is fair game. Like "I have faith that God died of a heart attack yesterday at 6:00 pm EST"
2. Personal god therefore evidence not required.
For a long time I thought this was untouchable. Until I saw Sam Harris do it here. http://www.youtube.com/watch?v=XjhbccXIp4c
He's absolutely right. You can easily make a mockery of the idea of a personal god. On any other subject, if a person said they felt something was true, in response to their lack of evidence, then their idea would be immediately discounted.
The problem of faith stems from a larger problem of the lack of understanding of evidence. What people don't realize is that everything that causes us to believe something is evidence. The light particles that enter my brain through the medium of my eyes and cause an internal image of a well defined and well known to me object are nothing more than evidence that such an object is in front of my eyes. If I had no such evidence, I would see no reason to believe that such an object is there.
For example, the only reason I believe that I am currently typing on a computer is that I can see an image of what I have been previously taught to be a computer in front of me. I can also hear the familiar sound of keys on a keyboard and I can feel what I have learned to be the feeling of typing. Furthermore, these all happen in the correct timing to lead me to believe that I'm not just imagining it all. If I didn't experience these things, I would have no justification for believing that I am typing on a computer.
To try to justify belief in god when the evidence is against it by claiming faith is therefore the same as to try to justify belief that I'm sitting on the beach in Bora Bora when all the evidence points to the fact that I'm sitting here typing away on my computer. Trust me, if I believed that faith could get me to Tahiti, I would definitely give it a try. ; )