1. A mobile payment system for effecting payment for a financial transaction in a payment processing system, the mobile payment system comprising: a financial transaction token including: an onboard energy source; memory coupled to the onboard energy source and configured to store and retrieve data regarding an account in the payment processing system; an embedded processor coupled to the onboard energy source and the memory; and an electrical interface in communication with the embedded processor; and a mobile subscriber unit including: an integrated processor in communication with the electrical interface; storage, coupled to the integrated processor and configured to store and retrieve code executable by the integrated processor; and a transceiver coupled to the integrated processor and an antenna, wherein the integrated processor, upon detecting through the transceiver of the mobile subscriber unit an authenticated signal requesting initiation of a transaction, executes at least some of the code stored in the storage to: (1) command, through the electrical interface, the embedded processor of the financial transaction token to assume an active state; (2) configure, based on one or more commands received through a user interface, the financial transaction token to select a particular account for use in the financial transaction; (3) obtain, through the electrical interface, data regarding the selected account; and (4) transmit the data regarding the selected account through the transceiver, wherein the financial transaction token is removable from the mobile subscriber unit and is programmable to initiate transactions in one of two modes, wherein a first mode includes completing the financial transaction by the mobile subscriber unit as a proxy for the financial transaction token, and wherein a second mode includes completing the transaction by the financial transaction token independently of the mobile subscriber unit.