I understand that carbonation might not last forever, and diet beverages don't taste good after a period of time, but I've always believed that the expiry date on the can doesn't matter, provided it's sealed and still carbonated when you open it, it should be safe to drink. I think a lot of people feel that non-obvious expiry dates (ex. dairy and meat obviously do expire) are a ploy to sell more products.
I've heard claims (see comments for some links) that the can or lining on the inside of the can will be eaten away over time causing carcinogens to be released into the liquid. I'm skeptical that the liquid itself would expire, but what about the aluminum can, does it expire?