The biggest impediment to recording a todo item is that it is inconvenient. I use that excuse to tell myself, "oh, i'll write it down later". Later never comes.
The fewer clicks to the "add a task" prompt, the less likely I can give myself that excuse.
90% of time management is mental.
That's why I recommend paper (no boot-up time), and PDAs like the original Palm that make it very fast (minimal clicks) to write down an idea.
A related excuse happens when I'm in the NYC subway. With no internet connectivity (2G, 3G, or WiFi), any great idea I have on the subway is destined to be not recorded, and often forgotten, if the app I'm using requires the network.
What would be optimal? A "record a task" button right on the phone. You would press-and-hold the button, it would wake up and say "Recording". You would then say your task and use speech-to-text technology to transcribe the idea. If the speech-to-text server isn't reachable, it should hold the audio clip until it can be reached; possibly doing the translation in the background.
The on-screen or physical keyboard should be available too, of course, but what I really want is a super smart, voice activated, task recorder.