This release fixes a compatibility issue when upgrading Calendar to 7.x-3.1 or greater. If you have updated Calendar, please update Calendar Tooltips as well (and vice versa).
Beautytips changed a keyword in their configuration: area has been replaced by cssSelect. Because of that calendar tooltips version 6.x-1.5 no longer works. See issue #847960. This 6.x-1.6 release fixes that problem.
Note that this version (6.x-1.5) is the first release officially hosted on drupal.org. You can find earlier releases on the following thread: http://drupal.org/node/462736