In order to avoid some confusion, I think there are two types of Cr-Mo steels being talked about here.
The tubing being mentioned is most likely a high strength type of Cr-Mo steel, 4130 for example, with about 1% Cr, and .2% Mo. I'm not sure about tubing, but I know in thicker sections it is water or oil quenched and tempered to obtain a high hardness with adequate tougness. It is often used as high strength tube, machine tools, oil field, etc. There have been several discussions on welding this type of material if you search the archives.
The other kind of Cr-Mo steels are designed to have high temperature strength and resist creep, as already pointed out. These steels start out at the low temperature end with only 1/2 Mo, and continue though to martensitic stainless steel. Some popular grades contain 1 1/4 Cr, 1/2 Mo, like 80XX-B2, or 2 1/4 Cr, 1Mo, like 90XX-B3, or 5Cr, 1/2 Mo, like 80XX-B6, or 9Cr, 1Mo and V like 90XX-B9. They are normalized and tempered in order to achieve their high temperature properties.