I believe Copenhagen has this too now. I visited in the same week they were launching the system (last year?) and it worked perfectly. All you had to do was press a button on your phone as you enter and leave the station and it automatically calculates what you owe. I loved it. It also worked throughout all of the different train systems.

Whenever I use that one I forget to check out then bike half across town several times over and at 3 in the morning I get a huge ticket.