Found this over on Wininfo. :)
As promised in late August, Microsoft has begun offering Addition Licenses for Windows XP, though only at a slight discount over the normal retail price. The company decided shortly before XP was released to manufacturing to begin offering families a way to upgrade to additional copies of XP through a new licensing program, originally called Additional Family Licenses. This license allows consumers to use their existing XP retail CD-ROM to install XP on a second machine by providing a new Product Key.
"As of [Tuesday], additional XP licenses are available at shop.microsoft.com," a Microsoft representative told me. "Additional licenses will be available through retail channels as well, as of [Wednesday]."
The licenses don't offer much of an advantage over the full price, however. Home Edition, for example, retails for $199.00, while the Upgrade version is $99.00. An additional license for Home is $189.00, or $89.00 for the Upgrade version. Prices for Professional Edition are similar. Pro retails for $299.00, or $199.99 for the Upgrade. But additional licenses for Pro knock the price down only slightly to $269.00 and $189.00, respectively.
News source: WinInformant