I should note here that the concept of God that I use here is from the Christian perspective. I was raised a Catholic and live in a country which is predominantly Christian, so the only concept of God I understand well is the Christian one.
I am an ambivalentist because it is my view that the existence of God should make no difference at all to how one acts in this life. If a person is a coward then the belief in God might make them refrain from evil and if a person is self-serving it might make them do good; to avoid Hell and obtain Heaven respectively. I hope and trust that my readers are neither cowards nor self-serving when it comes to how they live their lives. I also hope and trust that if God exists, he did not format the rules of life around the baser individuals who live among us.
A truly good person will do good for the sake of good and not because of any reward offered.
A truly evil person will do evil in spite of threats of punishment.
I have argued that in the absence of God, it is even more essential to be a good person: If I fail, if I succumb to temptation and there is a God then it is a tragedy for me alone. I assume here that God's good is infinite and therefore my own good which is finite does not add anything in terms of the global ratio between good/evil. If there is no God then my failure to do good means that the store of good in the universe is less than it would have been--there is a finite chance it is reduced to zero or less than zero (when evil is accounted for).
I often feel more comfortable around religious people than with atheists even though I (while not at all certain about it) don't think there is a God. I suppose the reason for this is that the religious and I share the belief that there is such a thing as good and evil and that there are severe consequences associated with how you act in this life. (What I think the nature of those consequences to be may be the subject of another posting)
So, getting back to the earlier point about why I think the term ambivalentist is lacking in some essential ways: It is great at expressing how I feel about the irrelevance of God when it comes to one's own choices in life. The question of the existence of God is very interesting from an academic standpoint. I would give much to know, with absolute certainty, if there is or is not a God. This aspect of the term is its large failing. If anyone out there has a better term, I would be interested to hear about it.
dbp Update: I didn't really mean to wait so long before making an update to such a loaded post, but things have been hectic... I am aware of the school of religious faith which posits that it is only through faith and scripture that it is possible to even know how to lead a good life. (I don't want to get into issues of religious fanatics: Atheist fanatics are just as bad and have killed at least as many innocent people). The evidence that I have seen from folks I know who do try and live according to the teachings of Christ, is that it appears to work: They have a sense of purpose, are living a good life and are happy. I do not consider this to be proof of the existence of the divine, only that there is nothing irrational about, nor counterproductive to having a belief in God.