I don't recall a "Diane".
During the trial, the alleged 'make-out' girl JA was peeping in on was supposedly from Phoenix, but was never named, IIRC.
I remember Linda Ballard, Lisa Daidone, and Deanna Reid, of course (all ex-GF's) but I don't remember a Diane/Dianne on that list of exes.
Just by way of reminder, here are some of the final messages from Travis to the lying torture murderess.
http://www.youtube.com/watch?v=kFejwEzV-cE#t=79
If there were any incriminating messages on the actual murder date, June 4, 2008, you can be sure that Juan Martinez would have introduced them.
Updated for the detail-oriented among us: IIRC, not all of these were text messages. Some of these messages were Internet chat (Internet Messenger, for example) messages. They were not all mobile carrier text messages which would have been composed, sent and viewed on their phones.