The same people are also working on re-using much of the hardware designed for the Pandora (the analog "nubs" are said to be very good) in a device called the iControlPad. See http://icontrolpad.com/.
Please note that the current hardware to uses Bluetooth to interface with the phone, rather than a physical connector since there seem to be licensing issues with that on (many/all?) of the Apple devices. Here's some video showing the iControlPad connected to a PC and used to control a game of Quake: http://www.youtube.com/watch?v=Gg7AtVuMqZU&feature=p....
ObPandoraJoke: I'm expecting my Pandora in two months, tops.
it would be as bulky as carying a GBA, but not as expensive as having another device.