Yeah, I figured AoO was a given since you'd be entering the other person's square. As for the ability to shake them off, that's a pretty good idea. I used the weasel's Attach as a basis for how to treat the climber. Though, keep in mind that anyone being climbed is going to be quite a bit bigger (2 size category difference at least, any less of a difference and it's just a hug), giving the bigger guy at least +8 modifier, not even counting his most likely bigger strength score.

As for the climbing DC, we figured a DC of 30 for the one time I've actually done this, as although handholds were readily available it was something motion, with a check needing to be made each round. Should be easy enough to have the DC increased if the opponent is actively attempting to shake the climber off (In the one case where I've used this, the bad guy was making no such attempts because he didn't consider it a threat)