Update: Somewhat unrelated, new post called How to: Delete photos from within Smart Albums in iPhoto.
For an entire year I've wanted to be able to do this easily. And I think I've now found the best way to do this without the possible side-effect of losing all of your photos on your iPhone.
One of the problems with the iPhone is that you can't delete all of the photos you've taken within from within the iPhone's interface. If you, like me, sync your iPhone photos with your iPhoto Library then you've no doubt noticed that iPhoto does let you delete all of the photos you are currently importing. But, if you've imported several times then iPhoto will only allow you to delete the current photos being imported.
So you're only option has been to create a new iPhoto Library, import all of your iPhone's photos again, and ask iPhoto to delete the photos when the task is complete. Or, you could click delete on every single photo on your iPhone one-at-a-time.
Solution: Use Image Capture.
My iPhoto Library has become very cumbersome. Opening iPhoto is a chore. Also, with the iPhone in its dock, iPhoto will compare its library with the one on the iPhone prior to allow you to import the new photos - and this takes a very long time. So, I'm going to show you how to use Image Capture, an application on your Macintosh that you may not even know you have, to create a backup of your iPhone's photos and, subsequentally, delete them from the iPhone - in one step.
Here are some photos of the process.
Delete the photos.
Empty camera roll result.
Depending on how many photos you have on your iPhone this process may take a little while. When the downloading is complete, the iPhone's photos will be deleted - permanently.
What if you want to save some of the photos on your iPhone? This can be done but it takes some work. After you've imported all of these photos into your iPhoto Library, create a new album in iPhoto called something like "iPhone camera roll". In your iTunes preferences for this iPhone ((You'll need to mount the iPhone again to see these options in iTunes.)) select this album for syncing. This way you only save the photos that you have put into this album.
I hope this helps those of you that want to delete their iPhone's photos in a quick and easy way without worrying about losing your photos!