@helidriver: fair enough, though the load capacity (and the load index, btw) scale to the volumnal capacity of the tire. This means that a larger tire in the same model line of tire will have a greater load capacity with few exceptions. It is unlikely that his OE tires have a greater load capacity than 2205 lb. The bridgestone desert dueler II's that are OEM on my Santa Fe (size 235/60/18) are load capacity 2205 lb @ max inflation pressure with a load index of 103. Seems pretty consistent, and a useful (but relative) guage for comparing tires and ensuring that they are not under capacity. My Mazda3 is an example of an exception in that the 50-series winter tires are XL (eXtra Load) to make up for their relatively low volumnal capacity given their load carrying requirements on the corners of a 3200 lb car.
I don't know where I heard it, but a good rule of thumb seems to be 2X the sprung weight for the corner of the vehicle where the tire is fitted. This takes into account weight transfer due to cornering, braking and accelerating, as well as the extra mass of people and cargo (assuming that the GVWR for the vehicle is not exceeded). As you pointed out, it isn't that simple but you have to start soMewhere in your calculations. The kind of use the vehicle will see (racing, business, pleasure, cargo) goes into it too.
To the OP: I have found that there's no great discernible flex in the relatively tall sidewalls of my Hakkapeliitta SUV winter tires. They ride hard anyway, and seem to be pretty stiff in the sidewall. Not likely that I'll be auto-x'ng the wife's SUV anyway! ;-)