I've been using iPhoto to download my iPhone photos to my Macintosh.  After every download, iPhoto gives you an option to delete the photos that you've just downloaded.  Sometimes I do, but other times I don't because I want to keep some of those pictures in my camera roll.

The problem is that if you skip this opportunity to delete, there's no way to delete photos from your phone using iPhoto.

Until Apple changes iPhoto, you can use an Apple application already on your computer, Image Capture that can copy and delete photos from your iPhone.