Since the promotional stills for episode 2, "Cloudy With A Chance Of Murder", have been released, Caskett fans have been in a tizzy about this particular photo's implications. Castle creator Andrew Marlowe clears up any questions and raises a few more.
This encounter is a consequence of Castle pretending to be single in order to keep his relationship with Beckett a secret from their co-workers. But that doesn't mean infidelity won't be an issue. An upcoming case will implicate Castle in the murder of an ex. "It makes everybody question: How well do we actually know this guy?" Marlowe says. But it gets worse: "Castle may have been having an affair with her at the same time he was dating Beckett," he teases. "That goes to the heart of all of Beckett's insecurities."
Source: TV Guide