As an Arduino/N900 user myself, perhaps I can shed some light on the situation.

Arduino is more or less an open-source microcontroller board with various functions that are as limited as the stretches of your imagination. It is programmed using "sketches" (programs) written in the Arduino development environment based on the open-source Processing language. Basically, it's an open-source microcontroller board that can be programmed to take any number of functions from the excruciatingly simple to the extremely complex.

It's certainly an interesting platform, and is great for hobbyists and those interested in electronics. Arduino can also fit a bunch of different needs. There are many well-documented arduino projects around the web; a quick search of google can elaborate further. The open-source boards are also pretty cheap, enabling realistic real-life use in many different scenarios. Any link that can be made between the N900 and an Arduino project would be extremely interesting.