Is it possible to link SVN repository files so that a file is actually a reference to another repository's file?

Yes, for directories anyway

See this section on svn externals. The svn:externals property is set on a versioned directory; its value is typically another versioned directory in a repository (possibly the same one) of your organization. They act in the end something like symbolic links.

Update: Good call also by basszero but note that the questioner almost certainly does not have an svn 1.6 repo, so he can't do it with files.

If you're lucky enough to use subversion 1.6, you can have external links for both directories AND files

Check out this post..
It helped me
how to properly set svn svn externals property in svn command line

For those who dont want to follow the link here is a brief overview.

svn propset svn:externals 'akismet http://plugins.svn.wordpress.org/akismet/trunk' .

• akismet - dir or file name you want your link to be called
• http://plugins.svn.wordpress.org/akismet/trunk - The link to the svn repo you want to link to
• . - The current directory so make sure your in the dir you want the link to reside

After running the above do a

svn commit

and

svn update