Wired Magazine broke a story that includes a memo "For Law Enforcement Only" that covers how long each cell phone carrier keeps information on cell phone calls and tracking. This includes information on where a cell phone has been (for ATT from 2008 to present) to billing information , calls made, text messages, etc. This can be useful information in many cases. For a PI in Texas you will need a subpoena to get this, but knowing how long the information is kept by the carriers is very important. You can see the memo here.
The story is about the court ruling that the government does not need a warrant to get cell tower information from the cell phone service carriers. This includes tracking the movements of the cell phone. The ruling basically says it is up to the district courts to decide if law enforcement needs a warrant. That is Federal Law. Texas state law requires a subpoena to obtain the records.
Ars Technica has a good article with detailed information concerning all of the stored infoprmation. They have an easy to read chart for your viewing pleasure.
The point is - knowing how long the data is kept by the companies can be very useful when investigating a case.
Remember to stay safe!