So if you haven't seen the MythBusters episode on this one, check it out.
Since I'm a fan of conducting independent experiments though, I did one on my own that backs this fact up - dark soda destroys rust. Why?
It's because of an ingredient that gives dark sodas their tartness - phosphoric acid.
For my experiment, I'd been given a set of Ren Faire leather armor, studded with copper disks that were badly rusting. I went to the dollar store and bought a jar of garlic, preserved in water and phosphoric acid. Then I removed the garlic, leaving a phosphoric acid/water solution, which, when applied with a Q-tip to the rust, took the rust away entirely. Yay - cheap rust cleaner! Ew - this is in the soda you drink!
What's a person to do?
Well, you know what I always say. Moderation in all things.