1. Plug your iPhone into your Mac and backup your device as you normally would.
2. Open Image Capture (just do a Spotlight search). Use this application to drag and drop your pictures and videos to a safe place on your computer.
3. Make sure iCloud is syncing your contacts with your Mac and the Cloud.
4. Now I am going to warn you, you will lose any application data, all text messages, and call history. FYI application data includes Game Saves, Settings, et cetera. Also I am assuming that you already have a backup of your music.
5. Double and Triple Check that your contacts are safely stored in the Cloud, and that all of your images are on your computer.
6. Make sure that your device is plugged into iTunes. Now hold your Home Button and Sleep Wake button for ten seconds. The moment you see the Apple logo (the device will power down first) release the Sleep/Wake button and continue to hold the home button. If done correctly, iTunes will detect an iPhone in recovery mode. Proceed to restore and download iOS 6. This method removes all traces of your jailbreak. If you were to take your phone to the genius bar, they would have no idea, and your warranty would be considered valid.
7. Set up your phone as a new device. DO NOT restore from a previous backup. That will leave residual files pointing to a previous Jailbreak. As such your device may exhibit some odd behavior. I only told you to back up just in case something went horribly wrong, such as losing your contacts. That happened when I restored my friend's phone. Luckily they were saved in a backup, and I avoided her resentment. (by resentment I am referring to a verbal asskicking

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8. Proceed to copy your media and applications over to your device. In terms of Photographs, I like to create an Album in iPhoto called Old Camera Roll (that contains the images and videos that I backed up earlier) and sync only that to my device. You can specify that in iTunes under the Photos tab.
Good luck. Feel free to PM me or reply with any questions. Enjoy iOS 6!