I agree Salem. My main reason for believing the text probably was not sent from Dylan's cell is that ER was seemingly unaware of it all this time-- despite the fact that she would have full access to his phone records from the provider.
I said yesterday, & still think it is possible that this communication could have been to someone Dylan didn't really know. Via internet gaming or something-- explaining why we wouldn't have heard of this communication before from family/friends. Although I do agree with other posters that it may be that this communication was to a minor friend of Dylan's whose parents requested privacy.
The idea of this last text being sent to a girl love-interest definitely caught my attention as well because when I think back to that age, that is definitely a good reason for why Dylan may have gone to all the trouble of texting from an Ipod or computer. That kind of communication would have taken some thought-- ("my phone is kaput, but I gotta send a goodnight text to her! hmm.. how can I do this?").. but a thirteen-year-old boy wouldn't hesitate to go to the trouble if it was for a girl!