After you click "Restore," iTunes will warn you that restoring your iPhone to its factory settings will erase all of the media and data on your iPhone. If you've already synced your iPhone, you can click "Restore" again.
From Liane Cassavoy, former About.com Guide
After you click "Restore," iTunes will warn you that restoring your iPhone to its factory settings will erase all of the media and data on your iPhone. If you've already synced your iPhone, you can click "Restore" again.