I checked in on-line on my cell phone in August. I was waiting for the train in frontierland when the 24hr mark hit. I had everything all set up in my cell phone ahead of time. It took about one minute, I had the boarding passes printed at the front desk at the resort when we got back for the day. I believe we had A1-A4. That was prior to the new boarding procedures, but they were testing it in Houston, so I was aware of the numbers on the boarding passes.
I have a verizon cell phone and I think the cost is like $5/month for the internet service. You can also add/cancel at any time. I was going to cancel when I returned from the trip, but it is a nice feature to have.