I\'m using a subversion repository and I want to know whenever somebody asks my repository for a checkout; like a \'svn co\' or an \'svn up\'. Is there a hook or some other
You could do whatever you want using Apache as SVN server and (for example) mod_perl to hook into the requests. Maybe the log files already show the information you want.
SVN via SSH should also be possible: Analyze the commands started by the SVN client and wrap them (for example, by renaming the original binary with ".orig" extension and putting a (shell) script into that place.
The following are all the supported hooks in Subversion 1.5, from the Version Control with Subversion book:
There is no pre-checkout or pre-update hook like you describe.
If you are using TortoiseSVN, you have access to client-side hooks.