Reader Warren Newman wrote to me with a perplexing problem. Every time he connected his iPod touch to his Mac, iPhoto would launch, showing the iPod touch's Settings app.
Although the cause of Warren's problem may seem obvious to anyone who has taken a photo with an iPhone, the iPod touch's lack of a camera muddied the issue for him. What happened is that Warren inadvertently pressed the iPod touch's Sleep/Wake and Home buttons simultaneously while in the Settings app, which takes a screenshot of whatever is showing onscreen.
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.
The solution is easy. Open the Photos app on the iPod touch, tap the Saved Photos album, tap the screenshot, tap the trash button in the lower right corner, and tap the big red Delete Photo button that appears. Alternatively, if you want to save the screenshot, import it into iPhoto when prompted, and allow iPhoto to delete the photo after the import.
As several people have pointed out, there's another way of thinking about this problem. Perhaps you want screenshots on your iPod touch to remain there, without launching iPhoto at all. To accomplish that, connect your iPod touch to your Mac, launch the Image Capture application from the Applications folder, and select your iPod touch under Devices. Then, at the bottom left of the screen, choose No Application from the "Connecting this iPod opens" pop-up menu. Quit Image Capture and no photo-related program should open in the future when you connect your iPod touch. (Obviously, this works with the iPhone too, though it seems more likely you'd want some application to import your iPhone photos regularly.)
It's a minor mystery, to be sure, but if you're completely not thinking about your iPod touch as a source of photos, one that could be rather baffling.