I'm having the same problem with 0021 under Ubuntu 10.04 Lucid and Arduino boards (both Uno and Duemilanove).
Serial communication from the Arduino board locks up the IDE and other serial monitors won't recognize the port - if I unplug the board the IDE comes back online. The IDE locks up so bad that I can't upload other sketches - I had to move to a mac (with 0021) to comment out the Serial.print(); in the offending code. Once that was removed I was able to go back my linux machine and everything worked fine.
Any ideas?