[In the credit-where-credit-is-due department, Liron's post was brought to light by a story in The Register, the cheeky British online tech site.]
The gist of Liron's post is that pretty much anyone can install an upgrade version of Vista on any PC, rather than having to spring for the full, standalone version. In the aforementioned case of Vista Home Premium, that means a saving of $90, since the upgrade costs about $160, compared with $250 for the standalone.
But wait! There's more (as they say on the late-night TV promos). Liron's suggestion might be of dubious legality. As he notes in his post, in the section entitled Is this Windows Vista Upgrade Secret Legal?, "I am no legal expert but I am guessing this will violate some part of the Windows Vista EULA." Which means, don't try this at home.
Fortunately, Liron's discovery constitutes mostly just an interesting technical diversion. For the majority of users, his technique is moot. Most people who are installing Vista upgrades are doing so legally, because they're (almost always) installing them on top of Windows XP. If you've got Windows 98 and you're thinking of using Liron's trick to save some money, I say, good luck to you, because Vista's not going to run properly on your hardware, anyway.
Liron's twist is essentially of value only to systems builders, and then only to those who are doing it on their own machines. (Presumably, if a builder is doing this on boxes he or she sells, Windows Genuine Advantage will ferret this out eventually, though I'm not 100% sure about this.) And builders using their own boxes are a very, very small percentage of the market.