I just don't buy Laurie suddenly deciding "wow...Dan sees me....I'm gonna go up there right now and sleep with him."
No, I think she's deciding that Dan cares for her in the here and now, while Jon's lost whatever love for her he might have had. That's a very important distinction, especially at the point Laurie's in at the time.
However, I will grant that this pill might have been easier to swallow if Laurie reacted as harshly to the breakup as she did in the graphic novel. Akerman plays the breakup as if to say "it happened, but it was a long time coming and that's that." In the GN, she's positively torn up about it, having lost her boyfriend/means of financial support for so many years. In the latter case, I could easily see Laurie in dire need of a rebound. In the former case, not quite as much.
To be certain, this plotline suffers from imposed brevity and from the direction Akerman and Snyder took Laurie. Still, I don't think it completely fell apart. It still works, just not as well.
EDIT: Or maybe Laurie decided to jump his bones because they just came out from a warehouse full of fun superhero gadgets. After all, the movie makes it abundantly clear that Laurie and Dan both have something of a superhero fetish (just listen to their talk about Archie near the end of the movie and try to count all the innuendos). Maybe an interactive tour through the Owl Chamber was enough to get Laurie hot, but Dan needed something more.