Yesterday Google announced the latest version of the AdWords API, version v2009. This is an upgrade from AdWords API 13 which was released in October 2008. There are major changes between v2009 and version 13 and Google wants you to be well prepared for the changes.
To get ready for the new version, you need to apply for the beta of v2009. You can however begin playing with the v2009 sandbox at any time.
You can read about the changes in the AdWords API upgrade over here. Version 13 will not sunset until early 2010.
Forum discussion at Google AdWords API Group.