My personal opinion is that tires (like virtually everything else nowadays) have a "built-in" expiration date. Simple reasoning: manufacturers want to sell you more of their product. In the olden days, stuff lasted forever. How can they prop up their bottom line if people only have to buy stuff when they really need it?

I bought a pair of Carlisle bike tires at a thrift store last year (2008). I know Carlisle last made bike tires in 1987. So, they were 21 years old, and perfectly fine. No dryness, no tread wear, no cracks. Tires I buy now last 2 or 3 years at the most. And tubes all seem to just "loose" air. I think it's just that the manufacturers have figured out a way to make tubes and tires that are designed to wear out so you have to buy more. IMHO.
Although, I've moved up to Schwalbe tires. Fairly expensive, but we'll see if they outlive the Taiwanese and Chinese brands.