Apple's standard policy for any repair or service is they are not responsible, at all, for your data. If they need to restore the device as part of their service they will do so. Their policy is that it is up to you to secure your data and they are not responsible nor liable for any data lost that may be incurred as a result of repairing a device. That policy is stated in their web site and applies to every single device they sell that has any capacity to store user data.
ANytime you you bring any device in to Apple for service, you should back up your data beforehand. If it's broken in such a manner you cannot backup, then Apple's response is still the same, as you should be backing up regularly and frequently. Your data is your concern, not theirs.
Just fair warning that they make no promise at all to safeguard your data.
see http://www.apple.com/legal/warranty/products/ios-warranty-document-us.html and scroll down to the section "YOUR RESPONSIBILITIES"