I am having the exact same issue with my iphone 3gs. After looking through the itunes help, I have yet to resolve the issue. So far I have reinstalled itunes and briefly disabled my firewall. I have also checked various options for ensuring the firewall doesn't block itunes but it makes no difference. I have even tried a different usb cable, as I noticed one website mention this as a possible issue.
If you find out the answer could you please post here to let me know too :-)