Something HAS changed since 2000. She got real big, and she started winning a bunch of Grand Slam tournaments. Compare a picture of Serena Williams in 2000 to a picture of her in 2003 -- and you'll understand why people have speculated that she has been using performance-enhancing drugs.
I heard a fascinating interview on a local sports radio station last year with a reputable physical trainer. He described the physical characteristics that he used to determine whether or not someone was using steroids, including a pronounced change in muscle mass in a short period of time (he claimed that nobody using "natural" methods could add more than 10-15 pounds of muscle mass during a typical off-season period) and a noticeable change in an athlete's head size and facial features.
He then rattled off a list of well-known athletes who were "on the juice," including baseball players Barry Bonds, Sammy Sosa, and Jason Giambi, along with Serena Williams. Anyone who still thinks Bonds, Sosa and Giambi were clean is delusional, as evidenced by the fact they showed up for spring training this year looking like toothpicks (conveniently, this was right after their trainer was indicted for marketing steroids).
If Serena Williams competes in the 2004 Olympics, watch how she magically sheds about 20 pounds of muscle mass before she shows up in Athens.