Hi,
I don't know if it is a good place to post it since it's a mobile application, but well, maybe have some time and know how to work with J2ME or similar to create a simple app to update
Twitter from the cell phone.
The API for
Twitter is here:
http://twitter.com/help/api (I think you need to be logged in to see it)
I know that Twitter has SMS support, but in some countries (like me in Argentina) we can't send SMS internationally.
It's better if it's done in J2ME because it can be runned in many cell phones.
Thanks