Hey Miso, what about one of those big gymballs? Those work for me. Seems to me you could semi-recline and do crunches w/o going all the way down. Tho my instructors keep saying to 'keep my bellybutton tight', I think its to protect your back.

From a pure physics point of view, I don't see how it matters how you set up to work abs so long as you're having to pull against some kind of resistance. I don't think you always have to lie down, just be pulling your body forward against a weight. I've also seen ppl standing up and pulling on those cable by bending forward at their waist. But I would worry about blowing a disc w/my back on those... personally, I like classes where an instructor can tell me if I'm doing something wrong.