I keep forgetting this so here is a short memo..
To enable the setting of $Id$ edit your ~/.subversion/config and remove the comment or add
enable-auto-props = yes
*.php = svn:keywords=Id
you can of course add other file types as well, the .php one was just an example.
After this you can run
svn propset svn:keywords Id file.php
to set the value for $Id$ in an existing file. (just remember to add the $Id$ and commit the file..)
Then when you check out the file again you will see somethin similar to this
// $Id: functions.php 22 2010-10-24 12:21:21Z henrik $
There is a lot of other nice things to look at in the subversion config, but this is a good start!