Digital Lifestyles has an interesting bit of information on the Pacemaker device.
Another handy traxxpad tutorial.
One thing a PDA device can’t (currently) do is to mix multiple input signals. I found one possible candidate for a portable mixing solution, the Samson Mixpad 4 which, from what I can gather, is almost exactly what I’m after. But of course they’re not being made any more.
So, if you know of any really small battery powered mixers, please let me know.