So, starting at 0000Z on 12/28/2006, Virgin Unite, the charitable arm of Virgin Group, is matching donations to the Wikimedia Foundation as part of its current fundraising drive. As part of the matching agreement, the Foundation agreed to run a small thank-you at the top of each page with a link to Virgin Unite's website. The thank you note went live at 0000Z. At 0012Z, Virgin Unite's website died.
Wikimedia quickly changed the notice to point to a thank you comment hosted on Wikimedia's own (far more robust) servers, and Virgin Unite's site is back up again.
The moral of the story: If you offer to sponsor a matching program at Wikimedia to bring traffic to your site, make sure your site can handle the traffic.