SVN is ignoring needs-lock property

Hi,

I'm using:
   SNV version 1.6.17
   CentOS 6.4

I have an XML file checked in with the svn:needs-lock property set. I
verified that the permissions on the file in the filesystem do not
include write permission.

As a test, I made a small change to the file and attempted a check-in
(w/o first locking the file). To my surprise, the file update was
applied!

Is this a known bug with the version of SVN I'm using?

Are my expectations incorrect? That is, am I doing something
incorrectly if I want a user to have to lock the file prior to check-in?

Here is the output of proplist for the file in question:

------------------------------ Delimiter BEGIN --------------------------------
$ svn -v proplist test.xml
Properties on 'test.xml':
  svn:executable
    *
  svn:needs-lock
    *
------------------------------- Delimiter END ---------------------------------

Thanks,

-- 
Mun