I think spokes and the quality of the wheelbuilding are actually more important factors in wheel strength than rims. Problems in wheel strength(going out of true, tacoing)(not stiffness) are a result of the spokes coming loose. Tighter spokes equal a stronger/impact resistant wheel.

And I may be wong, but I think hub width is also a factor in strength. Wider=stronger. Tandems have really wide hubs--some up to 160mm.