Hmmm, lots of input here. Well, I kind of poo-poo'd the comment about residue from SouthSound... but I think he's right. The first thing I tried was, I took a metal scrubby and scrubbed inside the mouth and down the sides, hard, as far as I could reach. Then I checked w/a flashlight inside - I couldn't see any change in the color of the metal. Or even scratches, for that matter. I'm sure they're there, but if the lining was as delicate as I prev. thought, it would have come off quite easily.
Then I used some Dawn and my plastic bottle brush and scrubbed the inside again, really well, and this pic is what it looks like now. (still wet) Under magnification I still see some curious spots where the 'sparklies' look like they've worn away. But all in all, aside from where the water inside is drying and there are obvious shading changes... I think I was wrong. I don't usually post full res pics but in this instance I believe it's warranted.
I owe CamelBak an apology and I have to edit the title of this post so no one else gets the wrong idea. I believe it WAS residue from the lemons or the cranberry I diluted for flavor. I really did clean it w/a brush the first time, but I may not have used soap. The appearance was SO MUCH like wear that I thought I was correct, and the interior was wearing off. Now, I believe I may be wrong.
Sorry to waste your time.