If I could afford it, I'd get the lens first. The reviews for the 300mm 2.8 IS indicates that it is an excellent lens for sports and wildlife photography. The crop factor, quicker shutter speed, expanded ISO and DIGIC 4 processor on your 50D are also all very useful for sports photography.
I've concidered purchasing the 1Dmk3 for myself, that is until a colleague recently purchased the 1Dmk3 from Adorama and had to returned it twice because of issues with continuous focusing and high number of missed shots. Last month he sold all his Canon equipment and purchased the Nikon D300 and 70-200mm and is very satisfied with the decision. Unfortunately, I now can't borrow any of his camera stuff.