iOS: Changing the signed-in iTunes Store Apple ID account
You can sign in to another iTunes Store account on your iOS device by following these steps.
When you sync your iPhone, iPad, or iPod touch with your computer, the Apple ID account last used to access the iTunes Store from the computer is synced to the mobile device. This Apple ID account is then used when accessing the iTunes Store from the device. This same account is also used for the App Store and iBooks Store.
Full details here http://support.apple.com/kb/HT1311
You can sign in to another iTunes Store account on your iOS device by following these steps.
When you sync your iPhone, iPad, or iPod touch with your computer, the Apple ID account last used to access the iTunes Store from the computer is synced to the mobile device. This Apple ID account is then used when accessing the iTunes Store from the device. This same account is also used for the App Store and iBooks Store.
Full details here http://support.apple.com/kb/HT1311