[Bilbo] forum user shared a nice piece of code to communicate to Arduino via Twitter (and Php, and Python, and offcourse a PC hooked up to the Arduino). If you need to get rid of shields and stuff you may want to follow his post:

Alright now I know that there have been lots of threads on this, but I think this is a different approach. I wanted to be able to control my arduino from my cell phone, and without a cell shield or ethernet shield. Thus, i did it through twitter through my computer.

via [ArduinoForum]