Wired has an interesting piece taking a look at why the Arduino has been so successful and a project like BeagleBoard has been less successful. While they hit on a of the few reasons the Arduino has been successful, the BeagleBoard comparison was flawed.

Though it was probably flawed for a reason: the BeagleBoard is about the closest open source computing platform available to compare to the Arduino... though it should be noted, it's not entirely open source. Unfortunately, it's also an entirely different class of computing device designed for different reasons and uses. It would have been more interesting to see a direct comparison of the Arduino with other 'open' microcontroller platforms to get more of an apples to apples comparison of what Arduino has done right vs. other projects.

Most of the other projects I'm aware of use 'open' as a marketing gimmick with some not very hard to spot proprietary right around the corner. There's nothing wrong with that, and if I were purely interested in producing a commercial product or add-on it would be a non-issue. But I've now invested far more than the initial $30 into the Arduino (funny how open source hardware ends up getting you to do that) not to mention the hours I've spent learning the platform and building hardware where I simply wouldn't have bothered with a more closed platform.

As far as the Arduino vs. BeagleBoard discussion, to me it's not an either/or decision. I now have multiple Arduino-compatible (or more correctly AVR-based) devices around the house and a BeagleBoard XM on order. I don't particularly see a lot of overlap or competition between them. In fact, one of the many uses I have planned for my new BeagleBoard is to act as the hub for my rapidly multiplying fleet of AVR devices.