Previewed at WWDC, launching in the fall.
Macbooks Tuesday, May 9? [Updatedx3]
Recent rumors have pointed to the Macbook being released in May, but no specific date had been given.
Update: (Moved to Page 1) In addition to receiving further confirmation of more Apple Stores receiving materials for a Tuesday product release, http://www.apple.com/ipod/ looks like it is spilling the beans with a reference to the as-yet unreleased Macbook. In case the site is corrected, a screenshot is mirrored here (emphasis added in image).
Update 3: Other reports claim the MacBooks will come in Black and White configurations.