Hi.
Whats your motherboard?
It´s a common issue with the new p55 platform. Asus have solved the problem and gues the others have to.
Try to update the bios to a later version. I did and it works fine after that with Itunes (latest version) and my Iphone 3g. (Asus p7p55d pro)
Good luck, have fun
// Zipander