While THASP is definitely onto something, think some of the people on here may be getting a little over-excited and making 2+2=5 in some cases. To take two examples:
Llodra has retired 31 (thirty one!) times in his career, and is known as a bit of a quitter. Plus if his tactic was just to evade the testers, playing in the doubles later in the day was probably not the best idea...
Federer's game (unlike some of his peers) is based on touch and technique, not fitness or his ability to recover from epic 5-setters. I'm no doping expert, but not sure how doping would have helped with that. Am open to correction, though...
Agree that there seem to be some serious doubts about certain players fishy players, but think there's a danger of conflating defeats/retirements with genuine issues.