do DT Swiss 240 hubs need proprietary tools to maintain them?
For the bearings you do need the tools. The hub is advertised as a tool-free maintenance hub, but that's only for light work. Check this post out on bikeradar.com:


  I recently e-mailed DT-Swiss on this
  topic and here's the conversation -
  
  Me - "This one has been mentioned by a
  couple of people - you advertise a "No
  tool concept" and a "No tool
  maintenance concept" while in actual
  fact only the drive mechanism is
  serviceable without tools and the rest
  of the hub requires expensive special
  tools. I've tried to explain before
  that bearing replacement probably
  falls outside of "maintenance" and
  enters the "overhaul" category but the
  quote "No tool concept" has being
  thought misleading. What answer do you
  have?"
  
  Them - "I've fielded many questions
  about the no tools concept too. It
  simply means that you do not need any
  special tools to perform normal
  maintenance such as ratchet cleaning
  and re-greasing. This should be done 1
  to 3 times (on average) a year,
  depending on your riding conditions.
  This is a 5 min procedure, even for
  the inexperienced. When you open up
  our tech manuals, the first picture
  you see is of a tool set, which looks
  scary and complicated. I can see how
  this seems misleading. The tools are
  required for more in-depth work such
  as bearing replacements. Want
  cartridge bearings? Then you need
  tools to install them. If tools were
  not part of the equation, then your
  bearing would simply fall out and the
  hub would be chronically loose. Show
  me a hub of any quality that does not
  require tools for complete
  tear-downs."


The tools are pretty expensive though.
http://www.amazon.com/DT-Swiss-DT-240-440-FR-front/dp/B001CK0S22

You might want to drop at the local bike shop.