Please keep in mind that the source of this news posting is a blog. Microsoft has not officially announced or commented on the discount. Mary Jo Foley blogged: "sources said that Microsoft will announce some time over the next few days that the company will allow Vista Ultimate customers to purchase two additional copies of Vista Home Premium for somewhere between $50 to $99 a piece."
Only if you purchase a retail a copy of Vista Ultimate ($399 USD) do you qualify for the "Family Pack" promotion. Let me emphasize that again: retail does not mean OEM. As a part of the Windows Anytime Upgrade marketing plan, a user would be able to use the Vista Ultimate DVD to install the Home Premium licenses on the additional computers. These "sources", however, cannot agree on the price of an additional copy: either $49.95 or $99.95. Even if it is the latter, that's still over twice as cheap (Home Premium retails at $240).
Let's do some basic math, shall we? If you purchase Ultimate and two additional Home Premium copies, you will be paying $600. That is much cheaper than buying 3 x Home Premium which would leave you dishing out $720. And you get Ultimate. However, you'll need at least 3 computers in the household that are Vista-ready or you won't really be saving much.