Some people are on texting plans so they don't pay for SMS. Oh and thinking about this some more, a really good input method would be voice-to-text services such as jott. Frankly, I can't imagine you can go too wrong having a lot of variety in input methods. From looking at sites that have done this right (eg. toodledo - check out the input methods they support, their syntax has some similarities to yours), the secret seems to be to have a robust mature API into your system, and then people will be motivated to provide the gateways with their favourite applications even if you don't.
Feel free to ping me when you have more to test-drive, my e-mail address is in my profile.
Feel free to ping me when you have more to test-drive, my e-mail address is in my profile.