I just finished reading the transcript to Bono's speech at the National Prayer Breakfast this past week.
I find it interesting that it takes a self-proclaimed "rock star" to point out to Christians that the purpose of giving and tithing is for performing social justice, and it's NOT ABOUT US. God is more concerned about the poor, the orphan, the widow, the sick, those in prison, those who have real needs, than he is about what percentage we give or how we might be blessed because of our giving.
Arguing about how much to tithe or when/if you do it is missing the point. God wants us to be the means of changing a world that is in need.
I think the best example of this in the Bible is in Acts 10. God answered the prayers of a Roman Centurion because he used his power and wealth to provide for the poor. It was because of his giving (not his tithing!), that God used him as the first Gentile family to become Christians. We have Cornelius to thank for bringing the Gospel to the non-Jewish world.