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.