Everywhere I go on google I get the same answer to this problem but it is the wrong answer for me. Originally I installed Subversion 1.5.6 for windows (i run Windows Server 2008
I had the same error. But the cause was I checked out the project with a svn 1.5 client and my subclipse plugin in eclipse was version 1.4. So updating my subclipse plugin to 1.6 version did the trick. so the problem was subclipse 1.4 does not work with projects that were checked out with svn 1.5 client.
Hope this helps someone.