First of all, I'd like to start with Sam's personality. If Sam liked Freddie, 'the dork', I don't think she would go as far to pull of an elaborate scheme as switching identities with her twin sister. That's crazy. A tough Sam would approach Freddie and just say it. Even if she would have chosen to go down that path and actually pretended to be her twin, Freddie would believe it wasn't Sam, and eventually Sam would have to come out and say she was really tricking him. She can't hide being Melanie forever. So sure, let's say Sam is really posing as her sister, and she comes out with it. She is hoplessly in love with Freddie. From any fan of iCarly, we know that Freddie has a beef with Sam. Freddie would obviously turn her down!
The second promo gives us way more information than the first, so lets focus on that one. Not bothering about the first part. Moving to the part that takes place in the Groovie Smoothie. Freddie takes a seat with Carly and Sam. He asks the blondie if she is either Sam or Melanie. Responding with a punch, Freddie sees that it is Sam. At this point, they most likely have a conversation. Then Sam has to leave apparently, because she is gone from the next scene.
In the next part of the promo, Sam is missing from the table. Freddie and Carly still sit, in the same clothes and positions, initiating that nothing has changed, except Sam is missing. Walking toward the table is a girl that looks like Sam. She is wearing a pink sweater and is carrying a pink bag as well. In the picture above you can see the scene is unchanged. Sam is missing, but Carly and Freddie still sit. Taking a closer look at the girl approaching them, we see that just maybe... this is Melanie, and not Sam.
This girl is wearing jewelry, a pink sweater, and is holding a pink bag. Now most people would know immediately, Sam would never wear pink. Even if she was pretending to be her sister. Of course the actor is Jennette, but Sam Puckett? Not likely. This girl seems nothing like Sam. I will be naming her Melanie. In appearance, it doesn't like anything like Sam. In the other promo, Melanie tells Freddie she thinks he is cute. What a polite girl. Sam? Definitely not a polite girl with manners.
Freddie thinks that this pink girl is Sam. Freddie asks her to go on a date, and Melanie agrees. Although it does never show, Freddie probably thinks that this cannot possibly be Sam. Sam would have declined immediately.
Okay, well, enjoy this episode. You can expect a review on this. Whether it is positive or negative, only time may tell. At this point, I'm betting... It's not Sam, it's Melanie.