I believe the posts/messages can be subpoenaed from Myspace and Facebook. They should be able to tell which IP each message came from, cell or computer. Even a cell phone has an IP in a sense.
And honestly, if Casey is like most other young adults her age, it's probably never crossed her mind to use that as an alibi. Most people that age knows it's possible to post from a cell phone. They probably assume everyone knows that. I only know because my 19 y/o daughter does it all the time.
The 6-7 digit numbers... I'm not sure if those would be these messages or not. My daughter has told me that she has to "log in" myspace through her cell and messages from there, so it is internet time, not really text messages.
Did that make sense? lol