Usually a sarcastic comment made after somebody pitches an idea. Otherwise, it can be said when someone is really excited about an idea that you think is stupid.
Person 1: "We could go play DDR at my house." Person 2: "...Or not."
Person 1: "Hey, you guys! I'm so excited about object!!!" Person 2: "...Or not."
what she said. a cool phrase.
Person 1: "Justin Timberlake is so cool. I mean he's got all the girls after him and he dances like wild fire. You've gotta say he's uber fit looking too..."
Person 2: "...or not"