An Apple patent filed back in 2009 shows the Aperture app running on a larger touchscreen -- something akin to the iPad, though, of course, it didn't officially exist back then. The image is actually part of a larger patent that goes through the whole process of how user interface elements work on touchscreen displays, but as you can see above, Aperture is clearly the app being shown.
Apple hasn't released that app for iPad yet, though it has released a few iWork apps and even a pretty comprehensive version of iMovie on the iOS platform. The iPad didn't even have a camera at launch, but now that there is a camera in the iPad 2, it's possible Apple could be putting together a high-end photo editor for the iPad's touchscreen.
Since this is a patent that's already a few years old, this only means Apple at least considered such an app at some time in the past. But it's not a huge stretch to think we'll see more complicated iOS photo software from Apple in the future.