This past June we were returning from Canada to Colorado in mid-afternoon IFR in our PA 32. The typical afternoon thunderstorms were in full swing. As we progressed homeward just east of a several hundred mile long line of Cumulonimbus buildups we heard Salt Lake Center clearing departing flights up to FL 260. Nothing higher. All requests for higher were denied with the same explanation-all the available (what there was of it because of the weather) airspace above FL 260 was already saturated with traffic. The controller was juggling airplanes as best as he could.