WordPress version 2.9 is almost complete and its release candidate 1 was made available for download yesterday. Enhancements and changes in this version are primarily focused on making publishing and managing your content more intuitive.
This article highlights important enhancements in WordPress 2.9 and provide resources that will help you quickly upgrade your themes or plugins to accommodate latest features.
http://www.w3avenue.com/2009/12/17/wordpress-29-enhancements-every-developer-must-know/