Yes to both, though it's very rare and depends heavily on the salience of the topic. I've actually changed someone's mind on abortion before, but in general, such kneejerk issues are almost impossible to budge someone on, at least all at once. The only times it usually happens for any issue of any significance is if you can get someone (or yourself) to think about something from an entirely new angle that they really hadn't considered before, allowing them to reconceptualize whatever it is you're talking about. It shouldn't really be a goal, though, because trying to do so only makes it more difficult.
Edit: ..and I pretty much just said the same thing as TJ, though he said it more eloquently. Damn him for thinking similarly to me.
I guess that's why I should read the thread first.