For those who do not know, Arduino is an open-source, embedded prototyping platform. The single board microcontroller hardware with a bunch of sensors and embedded I/O controls. Hardware components contains an Atmel ATmega328 AVR microcontroller, a thermister, a crystal oscillator, etc, although it varies from board to board. It is an advanced RISC microcontroller with 32 x 8 registers and a 2-cycle multiplier. The Arduino programming language is an extension of C++ - easily learnable. The source code of the entire software suite which includes an IDE is released through GPL. The hardware design is also available through creative commons for non-commercial use. Those who cannot do hardware can purchase a board from Arduino for a low cost.
Arduino's target users are hobbyists and artists, interested in creating interactive objects. But something tells me that the user can do much more than that. I can even imagine potential enterprenuers who have an idea about commercial electronic components can prototype their idea using Arduino.
Arduino first came into picture March, last year. But this year it's fame has grown up very fast, especially after BBC had an article and video on it by its technology correspondent Mark Ward in May. Although this article focused on how Arduino helps in relating the real world to the web world (which is cool in itself), it's just a part of it. Kudos to all the developers. You have done a great job in developing Arduino and making it open to the world.