I think I've got a good idea of how this can be pulled off with hl2.. even as a mod. (Especially looking at portal..)

Certain surfaces could be surrounded by a bush entity, like a trigger, that direct applied gravity and view. Doing this gives the mapper control over where, when, and how extreme the force is. Hell, just associating the player's view with gravity could make for some really cool sequences.. like if some device is pulling the player toward it, it would plant the player on his back or belly, making him helpless unless he finds something to grab onto.

A "hold player to surface" brush would direct this force to the face normal that the player is touching.. (i think that's the right terminology? im no coder.) It would look at the face's angle in relation to "0 0 0", and apply it to the player. I hope I'm making sense.

Anyway, using a brush entity.. the mapper could build this aspect into surfaces to.. let's say, 8 feet above the surface. This would give the player a means of escaping the gravitational pull, and being caught be an entirely other force. Imagine jumping from the floor, turning upside-down, and landing your feet on the cieling. Frickin' sweet.

Edit: I'm gonna make a diagram.. I'm getting myself all excited just thinking about it.