I think that it would be cool to be able to see your historical ranking within stackoverflow. The question is, would it be motivating to see the line go up? And would it motivate you, if you saw the line going down, to do better?
If, after a certain amount of time, your ranking in the system hasn't become a minor concern in your rationale for participating, I think that there's something fundamentally wrong. I'll grant you that getting on the front page of users and getting ranked higher was for a few months a big part of why I kept coming back, but I've resigned myself to never getting a higher ranking on SO and, likely, losing my position. This is actually a healthly thing as I was (and probably still am) spending way too much time on this venture. I'm not interesting in doing any more to encourage people to seek ranking glory.
Mostly now I'm mainly here to learn and to communicate what I've learned which, I've found, is actually the best way to actually learn yourself. You certainly find out how much you don't know when start answering questions on stuff you thought you knew, but didn't.
I think you've got a good point about it motivating people to do more / better. The question that remains to be answered, though, would be if it also made overall quality go down. I know personally it would make me want to do better (I admittedly kinda watch it as it is), but I'd sure not want people to bang in more crap just for the sake of having more in the pot to garnish rep from.
It would mostly be going up, because of the influx of new people with a one rating.
Maybe you could do it for all with rep over a certain amount, but I think the newbies coming in would still skew it.