Have a look at fantastick which is a great app which reports touch data from the ipod and can send drawing commands. It's meant to be for creating your own user interfaces on the screen but I imagine you can use it for creating visuals too, especially as it supports OpenGL.
Yes, fantastick looks truly awesome. With a bit of OpenGL experimenting and some familiarity with Max you can make whatever interfaces you want. Looking forward to a Share community building up around this app!
For running Max on an iPhone... maybe look into "rjdj", I saw it awhile back and it supposedly was able to let some PD patches run, which could be a nice workaround.