Initial release of Drupal 7 version. This is not compatible with the Drupal 6 version and there is no upgrade path.
The design of Peekaboo was changed between 6.x-1.x and 7.x-2.x. In 6.x-1.x Peekaboo uses a CCK field which acts as a sort of "field reference" to the field you want to include with Ajax. As of 7.x-2.x Peekaboo is purely a field formatter, and the removed functionality is now part of Field reference (http://www.drupal.org/project/field_reference).