Maria Emilia Salerni was junior #1 in 2000, and she hasn't done much this year, but I assume she just had a hard time going from juniors to pro, so I guess she'll have better results next year, same as Aniko Kapros, Virginie Razzano and Tatiana Perebiynis.
On the boys' side, Federico Browne from Argentina was the junior #1 in 1995 or so and now he's ranked below #300. Mariano Puerta was a French Open finalist and #3 and now he's ranked below #200 after facing a serious wrist injury.
I guess it's just hard to play in pros, specially after having success in juniors and knowing that everyone else is expecting you to win.