It's not at all clear to me that Mobile IP would be viable at the scale of a modern wireless service provider. It amounts to routing all traffic to/from the mobile device through a machine on the network of its "home" IP address. Without some fairly invasive routing shenanigans, this would be disastrously bad for users traveling far from their home network (e.g. a user gone on vacation).

Not that it matters, really. As far as I'm aware, there were never any substantial deployments of this protocol.