It's so weird to continually hear authors parrot the claim that the trivial cost of an OS upgrade is relevant to businesses. Fact is, any business over about 20 employees has a Microsoft Software Entitlement license, where you pay (substantially under market price) per user for Office, Windows, etc. And Software Entitlement is valid across all currently supported versions of the OS and Office. That means that if your company is running XP and Office 2003, it's entirely because your IT group didn't want to go through the trouble of upgrading everyone (and retraining users), NOT because of the cost. The time to test various enterprise apps and ensure they work and work reliably is expensive. Just because Apple wanted to encourage a bunch of bloggers to update their Airs isn't going to be a tipping point for Apple into any real market share - in fact it'll harm their chances in the future when they stop issuing security patches to a 2-year-old OS because "upgrades are free".
What is driving Apple's new-found fondness for free is that they don't want to deal with supporting older OSs. Unfortunately, that's a requirement for being in the business world. Apple has just excluded themselves from it.