You can't do this inside svn repository (unless you change this specific file on each commit, not very desirable).
However there is a small tool called svnversion (part of subversion commandline). By calling this inside your workingcopy (or pointing to it via argument) it returns the revision number of this specific workingcopy.
I suggest you call this tool after each update and write the result into a (svn ignored) textfile inside your workingcopy.
TortoiseSVN has client-side hookscripts (Post-Update-Hook) which can further automate this for you.
Keep in mind that svnversion returns more than just a single number, if you have sparse checkout, modified or switched working copies