Man I've been grappling with the grapple issue for ages, so hard to make a good one which has momentum. You could make a 'doodle jump' ish sort of game where you have to gain as much height as possible maybe set to sky scrapers so you can have moving platforms to grap and if you get past the sky scrapers you can maybe grapple on to birds or planes lol stupid idea.
Thats pretty sweet! Shows promise for cool idea As reference for the physics of it, I would recommend also finding a copy of an old Amiga/PC game called Fury of the Furries (not that kind of furry). One of the characters in it had a grappling rope thing and I remember it feeling really good in terms of weight and momentum. They also made it so you could wrap your rope around objects as you swang around them. Here's a vid but you should try it sometime for the controls http://www.youtube.com/watch?v=EqCaR6bNjVQ
Thanks guys, but I really wasn't going for using momentum, although quasi's link does make me want to try a few experiments. I was actually going for a jetpack kinda thing, so the only time that would work is when your not trying to swing.