BTHOOMT! (By the Hammer of Oh My Thor!)

I can already see Gossip Girl’s headline: “Spotted: The Queen Bee and the It Girl, Getting Catty at 30 Rock….”

Leighton Meester and Blake Lively, frenemies on The CW’s Gossip Girl, will appear in a November sweeps episode of NBC’s 30 Rock.

The increasingly stunt-happy, Emmy-nominated sitcom has cast the two young actresses as “former high school classmates of Liz Lemon’s in a flashback sequence that reveals a shocking and deeply ironic truth about Tina Fey’s morally superior alter ego: She was a Mean Girl!”

Meester and Lively join a roster of third-season-of-30 Rock guest stars that includes Jennifer Aniston, Will Arnett, and according to the blogosphere, even Oprah Winfey.

Photo: EW.com.