The theme is called Royale, and our friends at Microsoft New Zealand released it officially a few months back. At one point I downloaded it from Microsoft.com, but I can’t find it there now. This is where it used to live.
[Update: Even nicer than Royale (and very similar) is a theme called Energy Blue. It is designed for the Tablet PC, and found in this Tablet PC Experience Pack. The Experience Pack (at that link) only installs onto a Tablet PC. However, I used the instructions on this page to successfully install Energy Blue onto regular Windows XP. Thanks to Marcus Barton for the link!
Meanwhile, the link to Royale.zip on this page, which redirects to microsoft.com, definitely seems broken.]