If you buy at the end of September or early October (just before release), MAYBE you could get a coupon (slight maybe). I would imagine they'd want to give people some incentive so sales don't completely tank in the weeks leading to the leopard release. But if you buy in late July/August when the iMacs come out, they're not going to give you a coupon for an upgrade that's still 2-3 months off.
EDIT: as for Vista, that operating system was years in the making. It's a much bigger deal when a new Windows OS comes out, if only because it takes so damn long (6 years between XP and Vista). Apple, until now, has been having upgrades to the OS every 1-1.5 years, and it's not such a big deal to skip one OS and wait for the next. On top of all that, OS X is a lot cheaper than Windows. Vista's cheapest stripped-down-almost-not-worth-it version of Vista is still $100 for the upgrade, $200 new. The "media-center" version (home premium) is $160 upgrade/$240 new, the "pro" version (business) is $200 upgrade/$300 new, and the full, uncut "ultimate" version is $260 upgrade/$400 new. The full, uncut "ultimate" version of OS X is $129.