Well, I made one and I'm glad that it only cost $.64 for the materials ($.35 for a 2" 1/4-20 Eye Bolt and nut, $.29 for a large washer to use as weight/foot rest, string was free since I already had it). The results are less than impressive. The movement of the camera felt more restricted when attached to the string, but it doesn't appear to have made any real difference in shots that would conceivably be attempted without a solid tripod. I suppose it might be because I have pretty steady hands, but I had to go all the way down to a shutter speed of 1/6 second before I could see any difference at all between unassisted handheld shots and shots with the "string tripod". I don't normally have a use for one, but I wish I had access to a monopod right now just so I could take comparison pictures for this test. I don't think there's much need for tripod photos since we know what they will look like, but I'll post some if anyone is interested.
These pictures were all taken at 300mm zoom with my Fuji S6000fd at approximately 10 feet (I didn't measure exactly) from a calendar hanging on the wall of my kitchen. My camera does not have Image Stabilization, so none was used in this test.
Handheld at 1/15 second
String tripod at 1/15 second
Handheld at 1/6 second
String tripod at 1/6 second
Handheld at 1/2 second
String tripod at 1/2 second
Reference shot on a tripod at 1/2 second
Reference shot on homemade monopod at 1/2 second
Edit: Since I have it now, maybe I'll permanently attach the eye bolt to the walking stick I occassionally take with me on hikes for use as an impromptu monopod when I don't want to carry my tripod.
Edit again: Even if I don't take it with me, I'm going to attach the bolt to my walking stick so I can take those monopod photos I mentioned earlier. I'll do that in a few hours when I get back from shopping...
Edit one more time: I added a stable shot on a tripod at 1/2 second and one with my improvised monopod (eye bolt strapped to a 5 foot walking stick) also at 1/2 second exposure.