On tv.com - the notes for episode 17 are -
"17. Woman on the Verge
First aired: 5/12/2008
After Georgina reveals the true reason Serena left Manhattan, Serena goes back into her old habits. Her friends Blair, Nate and Chuck in order to help her, have to put their differences aside. Unfortunately, Serena is too ashamed to share the truth with Dan, therefore he assumes the worse about his girlfriend.
Meanwhile, Rufus is excited when his band is invited to perform at a Rolling Stone-sponsored concert, but Lily is the last person on earth he expects to see at the performance, especially since her wedding rehearsal dinner is the same night."
THEN
On tv.com - the notes for episode 18 are -
"18. Much 'I Do' About Nothing
First aired: 5/19/2008
Blair takes matters into her own hands to help Serena with the manipulative and evil Georgina Sparks. Lily while keeping Rufus in her mind, prepares for her wedding with Bart Bass that is designed to be the Upper East Side's social event of the year. Serena finally tells Dan the hole truth about her past and they try to work things out after Dan cheated on her, will it be too late? An unexpected guest shows up at Lily and Bart's wedding. Georgina goes somewhere where she can do no harm. Blair plans to go somewhere with Chuck."

I dont think Serena cheats on Dan next episode - but its obvious Dan goes and cheats on her when she covers up her lie by telling Dan that she has cheated on him!!!!!!!!!!