<p>Notice in the bottom right hand corner of this page it has the SVN revision id? I'm assuming that's dynamic.</p>

<p>I'd love to add that to some of my sites, just as a comment in the source to make sure code pushes are going through.</p>

<p>NOTE: You can also assume that the working directory of the site in question is an svn checkout of the repo in question.</p>

<p><b>Edit:</b> I'm looking for the global revision number, not the revision number of the file I'm looking at.</p>

Read up on Keyword substitution.  See
http://svnbook.red-bean.com/en/1.4/svn.advanced.props.special.keywords.html