I created a tutorial to guide you through the process of integrating Twitter with your Arduino without an ethernet shield. All you need is an internet-enabled computer. I hope some of you find it helpful. Feel free to offer comments or suggestions.
http://wellsb.com/post/5135881420/arduino-to-twitter-over-usb
Nice work! I have implemented communication in the other direction, may be useful to you Control an Arduino from a Cellphone (No extra hardware purchase needed) - #4 by system - Exhibition / Gallery - Arduino Forum . These methods work great together, for an arduino control + feedback system. I currently use it, in combination with a dockstar, to run several electronics in my house.
Wow, both of these ideas and implementations are great.
If you're looking for some place to document this for the world, maybe here?? It's a wiki and you could join and edit away.
http://arduino-info.wikispaces.com/HOME
Some examples of stuff here: (Click a Brick to see): http://arduino-info.wikispaces.com/BrickStarterSet
Or on the Playground here... But it should be grabbed...
Thanks!