By this point it is certain that you have either heard of or spent many hours of your life following this story. However the part that should hit home (regardless of which side of the debate you take) is that cell/smart phone data is becoming extremely relevant and necessary to review during an investigation.
If you have not included it as part of discovery, chances are you have missed crucial evidence, whether it be text messages, call details, location information, photographs, internet history, or application specific information (that you would never receive by subpoenaing a phone bill). With that said there are some important points to take note of during your request.
Nowadays encryption and security are common. If you do not have the device passcode, chances are the investigation will be dead in the water. Always ask for it! Additionally it should be noted that phones are not the be-all-and-end-all. Many times you will want to make note of the applications and cloud services they are syncing with in order to perform a complete collection of the evidence as opposed to a sync'd time period of data. Finally, if a phone is erased or destroyed all hope is not lost. Be sure to check for cloud based backups or computers the devices were syncing with. The files contained within these locations could be just as valuable as obtaining the phone itself.