I can confirm that changing the Image Capture setting in its respective preferences stops iPhoto from popping up.
God, was that annoying!
Seems like they should have some logical pointer in the iTunes/iPhone preferences for that.
Just like photos on the iPhone, screenshots on the iPod touch are stored in the Photos app, though in an album called Saved Photos (instead of Camera Roll on the iPhone). Whenever an item exists in Saved Photos, plugging the iPod touch into the Mac causes iPhoto to launch, since it assumes you want to copy the image to your Mac for safekeeping.If you launch the Image Capture program and pop into its preferences and set it to do nothing when a camera is attached, that should stop iPhoto from starting each time you plug your iPhone in.
Maybe in iTunes if you tell it to manually sync your iPhone that'll stop it from popping up.