This event got replaced by COMMON_PAGETPL_LOAD in devel on 2011-02-03 and will not be available in subsequent stable releases.
This event is signaled by pageTemplate() in inc/common.php when a new page is to be created (loaded into the editor), handlers can use it to load their own namespace templates (by replacing $data['tpl']) or process the page content (by changing $data['tpl']).
The following plugins are known to handle this event and their source code may be a good start for understanding and implementing a handler yourself.