if I flash a Duemilanove using IDE 0021 (with Ubuntu 10.04) I have this Problem:
I connect the Duemilanove to the Linux Box and the PC can receive the data send by Arduino. I reboot the PC with the Duemilanove attached, and the application on the PC can not receive any data because /dev/ttyUSB0 is in use. If I disconnect the Arduino and connect it again, then everything is working as expected. This happens only with Duemilanove, UNO (wich uses /dev/ttyACM=) is working fine.
If I flash the Duemilanove with IDE 0018 (on Windows) the Duemilanove is also working if attached during boot. So it must have to to something with IDE 0021.