So, why is Serena in Los Angeles filming scenes for Gossip Girl? Here's one theory, via TV Line, about the photos of Blake Lively and guest star Ethan Peck surfacing online:
"Peck plays a director’s assistant. Could it be Serena gets cast in a movie and starts crushing on him? In related news, Peck is in talks to return next season for an arc."
This would tie in (somewhat) to the arrival of S' relatives from the West Coast. Perhaps she and cousin Charlie take a trip to L.A.? Just one idea. We'd love to hear yours.