iPhoto prompts at the end of the import if you want to delete or keep the photos. As does Image Capture.

Easiest way to think of it:

The iPhone acts just like a digital camera with removable memory. When you take a picture, the photo is stored in the "Camera Roll", whereas a normal camera would save the image to the SD card. The camera roll is part of the backup the iPhone performs via iTunes or iCloud, with the idea being these are photos that have yet to be backed up any other way, as they are in the preprocessed/presorted state.

For syncing the photos back, the iPhone then acts as a viewer only. Photos are synced from iPhoto or a folder structure, using iTunes as the transfer mechanism. These files are stores back on the phone alongside the music and other content iTunes syncs, and not stored in the camera roll. iTunes will downsize the photos to a resolution that still looks good on the phone and allows zooming, while saving space over the originals. These files are not part of the iCloud or iTunes backup, since they already exist on the computer and could be resynced. Apple assumes the files on the computer are being backed up via Time Machine or some other method.

The workflow for taking a picture, importing it into iPhoto, then resyncing back to iTunes was getting a bit old. If I didn't kick off a resync in iTunes right after the iPhoto import, the phone wouldn't have my most recent photos. I'm going to try out Photo Stream and see how it changes this. It should guarantee that my last 30 days worth of photos are always viewable, and anything older I need to look for the events synced in from iPhoto.