jon skiied his forward of center at some comps last year so he could nose grab properly. but there is a line on the ski (the one farther back) that represents where you would tradionally mount a ski, if you just intended on doing turns with it. that mark coincides mainly with the sidecut of the ski. the mark at the middle of the ski, or the center mark, represents where the ski is fattest vertically (think even flex) and the apex of the side cut (think less energy than further back, but a more predictable turn shape) mounting your skis at center is more desirable from a park perspective, because you dont have to worry about compensating for your skis popping you unevenly, and your skis are more predictable if you want to set an edge of a jump, however, they will be harder to ski in powder, or crud snow because you will have more snow pushing up on the back of your skis, which will try to send you head over heels.
-CCR-
'every day should be a good day to die. dont lose your dreams'
                    --Dave Matthews
'uhh, hubert cumberdale, you taste like soot and poo.'
                    --salad fingers