There's an interesting Slate piece on the allegations of Marion Jones' insulin abuse by her ex-husband C.J. Hunter.

How is an athlete's performance aided by insulin, a substance more commonly used by diabetics to control their blood sugar?

Chiefly by boosting the body's supply of glycogen, a crucial muscle fuel. As diabetics know well, insulin, which is produced naturally by the pancreas, is a hormone that regulates blood-sugar levels by enabling the breakdown of glucose. The hormone stimulates this process (called glycolysis) by transporting glucose into muscle cells, where it is metabolized. If the muscles are flooded with too much glucose at once, however, they store the excess in the form of glycogen, a complex carbohydrate that provides energy to muscles during physical exertion. The more glycogen an athlete possesses, the longer she can keep her muscles pumping.