What version IDE is that? The versions prior to V1.0.1 had a bug in the ethernet library that would continuously spit out lines of characters over and over forever.
You can try one of two things.
Upgrade to IDE v1.0.1
http://arduino.cc/pipermail/developers_arduino.cc/2012-April/006444.html
or
patch the bug in your current version. Here is the report with the patch.
http://code.google.com/p/arduino/issues/detail?id=605