I've done a lot of hacking with MIDI on GP2X and other small and light Linux-using platforms, and there is no question in my mind that we'll be able to add support for MIDI to the Pandora pretty easily .. the class-MIDI drivers for Linux are superb, and as long as we've got a recent'ish kernel, it should just be plug and play. I just bought a USB->MIDI cable, very cheap one with not much but the connectors on each end, and it works seamlessly on all my Linux machines.
I'm primarily interested in the Pandora as a platform for music-software development, so count me in as someone who will be doing a lot of work in this area in the future ..