In the first part of this episode, when Noah and Alison are in Phoebe's house and she is getting ready to leave, she puts on the lilac dress and you can see it is still unzipped on her left side.
Noah then pulls her onto the bed and when she says, "god, you're so hot," you can see that the dress is fully zipped up in that shot.
Then when she immediately stands back up and leaves the room you can see that the dress is unzipped again.