Sportline help. loose piece *pics*

its probably cheap....just a piece of rubber

Mine was covered under warranty , but there isn't much to the part so i can't imagine it's expensive . You really don't have to replace it ....the only reason i noticed it was cause the car was on the lift..... never heard or felt anything . If you look there should still be some material under the spring still, the spring basically cut off the excess that wasn't really doing anything anyway.
EDIT:
It's called a bumper back OR a Jounce Bumper........... prices below are from GM parts direct

Bumper Back
05-09 $31.87

glue it back on, it doesnt do a damn thing hell rip it off lol...but seriosuly hit it with some epoxy and call it a day