After rewarding you with your highly deserved congratulatory statement, the game will give you a different ending sequence before the credits roll based on the time it took you to complete it from start to finish.
As an added bonus, after the credits roll and "The End" appears, if you press Start you'll begin the game again with all of your collected weapons and item power-ups, except energy tanks and missiles.
|5+ hours with Armourless Samus / 10+ hours:
Samus is one disappointed bounty hunter, showing you her back with her arm over her face.
Samus raises her left arm in victory, but her armor remains on.
Samus begins to flash, then her helmet disappears revealing her face and hair.
|Under 3 hours:
Samus will begin flashing before her entire suit disappears, revealing for the first time that Samus is neither a cyborg, robot or a man. She'll then wave before the credits roll. If you play through the game again after the credits finish, you'll be treated to a bonus where Samus no longer has her suit on (she's still just as strong, though).
|Under 1 hour:
Samus begins flashing again, and her suit completely disappears. This time she'll be wearing a bikini. She'll wave before the credits roll.