I mostly use the excellent subclipse plug-in for Eclipse, and rarely use the command-line svn client.
I tried to use the command-line client today with a project that had been checked out of subversion using subclipse, and found that the latest version of the svn client that installs by default on Hardy (1.4.6) doesn’t work properly with those files. I got the message “This client is too old to work with working copy ‘.’; please get a newer Subversion client”
After some rummaging around the inter-web, I found these instructions for installing the latest svn (1.5.1) : http://www.cylindric.net/blog/2008/07/29/installing-subversion-15-on-ubuntu-804
It definitely solved the problem, Thanks, whoever you are!