Evidently the reason that Michael Phelps is so fast has nothing to do with his 6'7" wingspan or his double jointed knees and ankles or his monster heart ... according to Alexei Koudonov at The Doping Journal in an article from 2004 titled "Doping by the pool" and comments in a Baltimore Sun piece from earlier this week from several other scientists, Phelps is guilty of "doping" by using his iPod on the pool deck to artificially increase his performance.
Previously published research (J Nurs Res. 2003 Sep; 11(3): 209-16) showed that in humans, music makes saturation of oxyhemoglobin (SPO(2)) significantly higher (compared with control subjects not receiving music therapy, p<0.01), and that as a result of music therapy the level of oxygen saturation returns to the baseline faster compared to the control subjects receiving no music, p<0.01), making it hard to detect the transient oxygen saturation shortly thereafter. The statistically significant higher SPO(2) level indicates the "enhancement of oxygen transfer" and implies that "music by the pool" is prohibited by The World Anti-Doping Agency (WADA) in competition blood-doping method of "the use of products that enhance the uptake, transport or delivery of oxygen", apparently conflicting with the essence of olympism, and WADA calls for "ethics, fair play and honestly".
If you didn't feel like wading through that - the basic summary is that there is some research in human infants that shows that listening to music might help the blood carry more oxygen for some indeterminate period afterwords. How long that period of extra oxygen carrying capacity lasts could vary from seconds to minutes, no one is sure. In his original article, Koudonov opines that Phelps listening to his iPod is tantamount to using a product specifically designed to increase his performance.
So is Phelps guilty of "doping", or is he just a guy trying to get himself focused before each swim by blocking out distractions? I would say that the second factor has far more impact on his performance, given his legendary mental toughness and focus and the extremely minute effects of the alleged "music doping". Each athlete has their own way of "getting into the zone" and Phelps seems to do it through listening to rap. The absurdity of claiming that using music before a swim is "doping" makes this just sound like this researchers and journal were trying to find any way possible to invalidate Phelps performance.