On my iTouch I almost always use my right index-finger exclusively in portrait-orientation, and have got used to tapping at a decent speed with it (with all auto-correction turned off, as I found that was more trouble than it was worth-- much better to tap correctly in the first place).
I rarely use the keyboard in landscape mode, but I remember when I had a Psion 3a with a proper keyboard that I always used the two-thumbs typing method with it.