Some people on the Subversion Users mailing list have requested the ability to perform actions (such as sending out an email) when someone checks out or updates a working copy. Since Subversion provides no post-checkout or post-update hooks this isn't as easy as it could be.
Fortunately as of Subversion 1.3.0 there's a new feature available in the mod_dav_svn Apache module which logs only high-level events like checkout or update. Furthermore Apache lets you log to a process instead of logging to a file. Combined with the right script, this lets us implement the post-checkout and post-update hooks. svnhookdispatcher is that script.