Oh no,, it's all gone horribly wrong. I followed these instructionshttp://arduino.cc/forum/index.php/topic,52447.0.html
very carefully up to the point UPLOAD THE NEW FIRMWARE (console commands)
The first upload instruction responded with dfu-programmer: no device present.
which doesn't look good. Am I being stupid? The UNO was plugged in with the little TX light glowing.
I then tried to verify a simple sketch from the IDE. I got an eror: Cannot run program "avr-gcc" .... NO such file or directory. From this I deduced that my IDE had gone west, so I decided to reinstall it, starting with arduino-core. I went to Synaptic and searched for arduino packages - zilch. I have Lenny installed but there are no arduino packages available, apparently (I checked on the Debian website).
So now what, step by step, do I need to do to restore the situation? The Linux installation instructions on the Arduino website are confusing me.