The same happens to me!
Upload works 1 time out 7-9 times, serial monitor never (I use gtktty).
I use Ubuntu 10.04
mune@lello:~$ uname -a
Linux lello 2.6.32-25-generic #44-Ubuntu SMP Fri Sep 17 20:26:08 UTC 2010 i686 GNU/Linux
and, despite the CPU is an AMD 64 dual core, it is a 32 bit system (more software).
The only non-standard thing is that I use already a USB-serial converter (/dev/ttyUSB0) to communicate with a little UPS; the system for my arduino uno creates a /dev/ttyACM0 device.
I use the latest (0.21) IDE
mune@lello:~/Apps/arduino-0021$ ./arduino
Experimental: JNI_OnLoad called.
Stable Library
=========================================
Native lib Version = RXTX-2.1-7
Java lib Version = RXTX-2.1-7