According to ZDNet (link dead), Microsoft plans to offer cheaper version of Windows Vista Home Premium edition for somewhere between $50 to $99 a license, with each licensed copy Windows Vista Ultimate entitles to purchase two additional copies of Vista Home Premium. The can called as “Family Pack” discount or promotion, the customers will have to purchase and own a retail version of Windows Vista Ultimate license, which has a recommended retail price tag (RRP) of $399. OEM version of Vista Ultimate comes preloaded on your branded PC from Dell, HP, Lenovo and so on will not be entitled for this offer.
If you purchase Windows Vista Ultimate, the DVD that you get will contain multiple versions of Vista editions, as part of its Windows Anytime Upgrade marketing plan. So, if you decide to take advantage of the “Family Pack” promotion discount offer, you’re able to “unlock” the additional licenses from the DVD using a software serial key or product CD key after paying for the additional copies by credit card.
Update: Windows Vista Family Pack official terms and prices.