Man, what a can of worms I opened! ;)

PVC yellows with age. Period. You can slot down the process with many of the methods that Ron mentions (good post by the way), but they WILL age. The yellowing shown in the first post was uneven, so it was obviously exposed to 'something' to age it quicker. Yellowing during storage 'should' age evenly.

Now, all cards are NOT created equally, so this is a variable that will be harder to quantify. There are various levels of quality of PVC (Co-polymer, homo-polymer blends, etc.) that can made into cards. The quality of the plastic will factor heavily into the longevity of the card. Not only does PVC yellow with age, it also gets more brittle - so handling will eventually become an issue as well.

I'm at work right now - this topic may be better suited for a 'roundtable' at the convention, no?

In my opinion, the best solution(s) for storage are PVC-free pages stored in a binder or paper envelopes. Here at ACC, we have cards stored in both manners, and haven't seen a difference in either method (cards stored more than 10 years). Acid in paper typically should not harm PVC (non-porous) as it does other paper items. PVC is more likely damaged by leeching from other plastics, exposure to UV light (sunlight, etc.), or exposure to chemicals (acetone, etc.).

Of course, you could always SLAB them! :0)

(above comment was NOT serious, but since the worm can was already open, why not a little more fuel?!?!?!)