They've been putting those vouchers in the boxes of computers or OS retail packs for several years, and they have never in that time meant a thing that I know of. It may be in case they decide to offer an upgrade price, but at the $129 or $139 regular price (I forget which it is) of the OS, they don't offer an upgrade price.

I've seen dozens of those proof of purchase voucher things (there's usually a card with three of them, I think, something like that) from buying new Macs, and I've never needed one for anything.

There is always a cutoff date for a way to get the newer OS version, with a $20 fee for mailing you the DVD, but has not gone back in time for a period of months in many versions. The last couple versions basically it was only there in case you bought a Mac or the previous OS on the day of the new OS on-sale announcement or later. (There would still be lingering inventory of old boxes at third-party resellers and such.) The on-sale announcements have typically been "available today", so there's really no leeway. Buying the OS now wouldn't even have qualified you for an upgrade if it was released in June, based on past patterns, let alone October. Some will argue that this is lame, but I also look at it that the OS cost is relatively cheap. If they suddenly started charging $200-300 for the retail OS, then upgrade pricing would be in order. At $130, it's not that bad. They do give a significant education discount to $69.