I still assumed they had sex even though that scene was taken out.
I can't help but feel like her motives with brendan aren't so clear cut. sure, she's using him, but she also has feelings. she mentioned how she noticed him, despite his best efforts to make sure no one did. and you can sort of see her feelings throughout, especially in the end.
I actually didn't get the feeling that Laura was really into him at all, but that she really was just using him. She just knew what to say to manipulate him to try to get him to trust her. She knew that Brendan was going to try and find out exactly what happened and she was doing whatever she could to get him under her thumb.
And if they did sleep together, I don't think it really meant anything to Brendan. He was obviously emotionally distraught at that point and was thinking of Emily when Laura came to him, and I felt that maybe he was even using her in a sense, for comfort. It was the first time in the film he couldn't hold up anymore and let himself break down and cry over the death of his girlfriend. And I'm not sure exactly how much he had figured out at that point, because Laura had already stolen the brick by that scene. And even if he didn't see the cig until after the sex, she was still probably a definite suspect to him. I don't think he ever once gave his trust completely to her, but instead just made her think he trusted her and then was able to use her to help him (rides, watching his back, etc.) without making her think he suspected her. Because he did always tell Brain to stay away from her, because I think he knew just how manipulative she was.
Just my opinion, I think I need to watch the movie again...