Assuming you don't have AppleCare+, the conditions that'd void it that I know of are:
- Sending in a jailbroken device
- Dents
- Broken screen
General conditions for servicing without AppleCare+ is that damages to device caused by accidental damage is not covered,mse does jail breaking the device. With the plan, it allows for 2 cases of coverage for accidental damage, subject to a charge.