Rachel Bilson (Summer Roberts) and Wilson Bethel (Brad), would go on to star together in the TV show 'Hart Of Dixie', playing Zoe Hart and Wade Kinsella respectively. Their characters are love-interests throughout the show.
The discussion that Kirsten, Sandy, and Seth have about Sandy doing "Grease: the Musical" in college is an inside joke. Peter Gallagher performed in Grease on Broadway.
Summer's exaggerated description of her fears for her and Seth's relationship somewhat accurately predict the path of the highly-media covered relationship of the actors playing them, Rachel Bilson and Adam Brody. However both have later found more lasting relationship success.