Back up only camera roll via iTunes?

You can import the photos from the Camera Roll to your computer, deleting them once the import is completed, and then sync them back to the phone via iTunes. You will need to go the the Photo tab in iTunes to select what is to be synced from the computer. Once that is done they will no longer be part of the iPhone backup but they will be held in a minimum of two places (the phone and the computer that you imported them to) and hopefully 3 or more places assuming you backup the computer. They will no longer be in the Camera Roll but will be in the Photo Library which is accessed in the same way as the Camera Roll. Doing this also gives you more ability to organize the pictures and reduces the backup size.