While I can easily find the binaries for the latest version of subversion (1.6) I can\'t find the binaries for Ubuntu (or Debian).
The download page at tigris.org sugges
You don't need the 1.6 version to work with your Windows working copies. SVN is clever enough to recognise the features provided by clients, and only allow those features through - so your 1.5.4 client can connect to your 1.6 server without any problem; as can your Windows 1.6 client.
The only time you'll have problems here is if you're using 2 clients with a single working copy - ie your Windows and Linux clients both commit and update a single, shared directory. Such an arrangement is a bad idea anyway, you'll have all kinds of problems even without counting EOL differences. If you have such a situation, checkout a new WC, so you have one for Linux and another for Windows.