Ever since Mootools 1.2 was released in 2008, many Joomla! developers have faced the dilemma of whether to upgrade by applying a unofficial hack so as to enjoy the improvement provided in the new version or stay with the default Mootools 1.1 to ensure stability and compatibility. All of that will change when Joomla! 1.5 with Mootools 1.2 is released on July 15, 2010.
The coming Joomla! 1.5 release will include a plugin which, once activated, replaces the core JHtmlBehavior class with its own implementation that enables Mootools 1.2. A compatibility layer will be loaded with Mootools 1.2 to allow legacy applications written in older Mootools 1.1 continue to function. However, in case of problems after enabling the plugin, administrators can disable the plugin and revert back to using Mootools 1.1.
You can read the original announcement and download the code to try it out.