Although this may sound like an antithesis to our goals, it is possible to have an Arduino board send tweets without directly-connected Ethernet hardware. To do so, Ian Foulds developed a method using a USB-connected Arduino and a PC. The computer runs python scripts that listen for text from the Arduino, and another is python-twitter which sends the text to the twitter service. The main reasons for using this method would be to reduce the Arduino-related hardware size as much as possible; or if you don't have a wired internet connection - as the Arduino board makes use of the host computer's wifi connection.
Finally, for most users there's no extra cost - so give it a whirl, follow Ian's instructions and get yourself a twitter account. Then you can also follow us on twitter and Google+ for news and product updates as well.
If you're looking for the smallest piece of Arduino-compatible hardware to use as part of an automated tweeting machine, consider our Freetronics LeoStick. Apart from being one of the smallest Arduino-compatibles on the market with USB, it also has an onboard RGB LED and piezo for sound and knock detection: