I think it has potential - obviously in portrait mode more than in landscape.

The only thing is that not much on the N900 actually works in portrait mode!

For example I wish conversations would work in portrait, because it's when writing text messages that it's really useful to be able to do it one handed.

I think the most relevant test for your system would be to compare it with T9 input, as I reckon T9 might be quicker than one-finger qwerty typing.

Definitely worth pursuing your idea though.