I don't think the OP is proposing to phase out the current method of navigating and selecting text, but rather adding an additional shortcut for people who do a

I don't think the OP is proposing to phase out the current method of navigating and selecting text, but rather adding an additional shortcut for people who do a lot of text editing. So the question you have to ask is "will this get in the way of beginners and/or casual users?". Precisely because it's so unobtrusive and non-discoverable, I think the answer is "no". Like the multitasking gestures on the iPad (which will never entirely replace the home button), this would be a complement to the intuitive, consistent and metaphorical (but inefficient) method of selecting text currently. In fact precisely positioning a cursor is very nearly unworkable as a direct-manipulation task, since the target is so much smaller than a fingertip.