My current panorama mount
When I use a tripod to take panoramas, this is the thing I usually use (as of January 2009). It cost me about $4 in parts, and took about an hour to make, and it works just fine. Of course, when I want to use another camera/lens combination, I have to measure, experiment, and drill another hole, but I don't get lenses very often.
It can shoot straight up, straight down, anywhere you point it. The screw on the right you can loosen, and swing the camera around to get a less obstructed view straight down at approximately the same height as normally. The cheapness of my tripod makes the height less precise, though, since the weight of the camera makes it flex and nearly tip over.
(lighting: open window on the left, sb-800 above right, pointing down, kind of wish the window light was stronger)