Logically, it would be exactly as arrogant to presume that God exists as it would be to presume that God does not exist. So theism is just as arrogant as atheism in that regard.
However, most theists (perhaps not "Piculet") do not presume to know that their God exists, so much as they choose to hope and trust (place their faith in) the idea that the God of their understanding does exist. So that they can then gain the benefits of "living as if" it were so. This is an advantage that atheists do not have, and that most atheists are unable and/or unwilling to even recognize. In that sense, I feel that atheists are being unreasonably 'arrogant'; though they aren't really harming anyone in this arrogance, but themselves. As opposed to those somewhat rare, but annoyingly loud and arrogant theists who falsely presume to know that their God exists, and then try to force that presumption on everything and everyone around them.