I'm not cheap. I pay for all my software. Suggesting to Apple that OS X 10.8 Mountain Lion be a free update isn't about saving $30 bucks. I've gladly paid for every OS X update and will continue to do so if that's the direction Apple takes.
But I think the best idea from iOS, free updates, should come "Back to the Mac". This policy would allow developers to use the latest APIs as quickly as possible and help developers & Apple reduce support cost for older OS X versions more quickly. Any kind of monetary transaction is going to slow down OS X update adoption. Any iOS developer with access to analytics can see how quickly iOS 5 adoption has taken off (I've seen 60-75% already). Lion has sold 17 million copies, Apple's best selling release ever. The last reported install base was 58M Macs. Lion is on roughly 29% of all Macs. Pretty good, not nearly free iOS 5 adoption good.
17 million Lion copies is a $510M gross, no small stash of cash to throw away. Except Apple has nearly $100B (or more by now) in cash, and making up that revenue is only 398k Macs at an average selling price of $1281.
Imagine the ad Apple could deliver promising free software updates for 2 years on all their devices! Contrast that with the Android 4.0 upgrade schedule from Motorola or the confusing array of costly and confusing Windows upgrades, the comedy gold practically writes itself.
I think Apple replaces the revenue easy with a combination of Apple's cut of increased Mac App Store sales, decreased support costs, but more importantly new users. They could really amp up the expectation that when you buy a Mac, Apple "takes care of you" unlike with all those razor thin margin PCs. More importantly, it hits Microsoft where it really hurts, their wallet.