Both the board and sled will be running through, hopefully the shims will prevent the planer from flattening the bow or curve. I've read online where people have got this to work.
Yes, it definitely works. Of course, the more severe the warp/ twist/ bow, the more complicated the shimming will be, but jointing such a board using a wide jointer wouldn't be a picnic either.
I use playing cards as shims. Finally playing with a full deck! Actually, I've used as many as two decks to really get things tight. A couple of dabs of hot melt glue to secure the board to the sled and you're in business. Just remember to take light passes. It wouldn't hurt to wax the sled, so the hot melt glue comes off easily.
My planer sled consists of multiple layers of MDF, based on a design in PWW. The idea is that a thicker sled will help withstand the pressure from the rollers, so that the sled itself doesn't warp.
I think it's overkill. The 3/4" sled I made from laminated particle board (Matt posted the picture) does just fine. Glue comes off easily as well. And it's a LOT lighter.
If you have one corner that's really off, a hand plane does wonders. I learned that trying to get one of Jeff's diving boards into shape. I now even use that technique with narrow boards that fit on the jointer, it's a lot quicker to shave down the worst section by hand instead of making 26 passes lifting the board.