After having read about it for a _very_ long time, I finally got around and bought an Arduino Uno the other day.
If you do not know what it is, all I can tell you is that it is an open source hardware for learning and experimenting with electronics and software in an easy way. It has a very big supporting community, and a very good website for all levels of users. Check out http://arduino.cc/en/ for more information.
To setup the Arduino IDE on my Dell laptop running Ubuntu 10.10, I followed this guide, http://www.pluggy.me.uk/arduino-ubuntu/, and it worked straight away.
The Arduino IDE comes with many examples, which are quite nice to use in order to quickly learn how to use a certain feature. Doing the standard 'Hello world' of electronics, making a LED blink, was done within minutes, thanks to the examples and guides.
Next up will be to use it with Python...