As with all Metroid games, you'll receive a different ending after the credits roll depending on how long it took you to complete the game.
SPOILER WARNING!
Don't read on if you want to see the endings yourself.
5+ hours:
When "The End" appears and your total time is displayed, Samus continues to run towards the screen.
3-5 hours:
Samus leaps up out of the screen and lands with her armour still on, she then stands motionless.
Under 3 hours:
Samus will again jump up out of the screen, but when she lands her entire suit will disappear revealing her wearing a two-piece bikini. She unties her hair and stands motionless.