How to “reset” an Arduino board?

I've uploaded a sketch to an Arduino Uno whose loop is something like this:

void loop(){
    Serial.println("Hello World");
}

So, now, I can't upload anything anymore, because the IDE says "port already in use".

Is there a way to "reset" the Arduino without another programmer?

EDIT :
Nothing else is using the serial port, and everything went just fine until I uploaded the previous sketch.

EDIT :
I've found some interesting things:

• Problems with 0021 on Ubuntu 10.04 Lucid Lynx
• Re: Problems with 0021 on Ubuntu 10.04 Lucid Lynx
• Uno sending serial data becomes un-connectable to Serial Monitor