Is there another Arduino ethernet module besides the "Arduino Ethernet Shield"?
There is also one from Freetronics, which works with Arduino and I have also tested it with Netduino.  You can also get a nice PoE adapter from them.  Their shield is also based on the Wiznet 5100 chip, and has incorporated the following design features to hopefully help with any issues with the Arduino Ethernet Shield (taken from their website)


  SPI Fixes
  
  Combining Ethernet with other SPI
  devices can be really tricky because
  the Wiznet chip doesn't relinquish the
  bus properly when it's deselected. To
  fix that problem we slaved the
  Wiznet's SEN (SPI Enable) line to the
  CS (Chip Select) line, which means
  that whenever your sketch deselects
  the Ethernet connection in order to
  talk to another SPI device it will
  work exactly the way it should.
  
  No more messing around with cutting
  tracks and other nasty hacks you may
  have seen mentioned on the forums.
  
  Reset Fixes
  
  We've also slaved the Ethernet
  Shield's reset line to the Arduino
  reset line, so if your Arduino is
  reset the Ethernet Shield will
  automatically reset as well. The
  Wiznet reset line is also held active
  for long enough to make it restart
  reliably each time the Arduino itself
  restarts.
  
  Power Filtering Fixes
  
  Ethernet connections are very
  susceptible to electrical noise, so
  the Wiznet chip has multiple ground
  pins on two separate buses and they
  need to be individually decoupled and
  the buses linked by an inductor. We
  took care of that by implementing
  proper decoupling on the power and
  ground rails, ensuring you get maximum
  reliability even in electrically noisy
  environments.