The mobileOK Basic Tests 1.0 document is based on a default delivery context (as defined in the Device Indepence Glossary) that content providers are expected to base their responses on when an unknown user agent requests content from their servers.
This default delivery context is detailed in the Mobile Web Best Practices, developed by the Mobile Web Best Practices Working Group.
The W3C mobileOK checker service fetches pages from the Web to verify how mobile-friendly they are; when it fetches these pages, it shows in the Web server logs under the name “W3C-mobileOK/DDC-1.0 (see http://www.w3.org/2006/07/mobileok-ddc)”, since it is supposed to emulate the delivery context defined above.
Seeing this address in your Web logs is usually simply the sign that someone used the mobileOK checker to check on of your Web pages.