Buffy alum Michelle Trachtenberg is joining the cast of Gossip Girl, reports TV Guide.

Trachtenberg, 22, will play Georgina Sparks, a troublemaker from Serena’s (Blake Lively) past. Mischa Barton reportedly turned down the role.

You just know there’s going to be a Blair (Leighton Meester) and Georgina showdown!!!

Michelle’s comeback career since 2003’s Buffy is certainly starting to take off! She’s also starring alongside teen heartthrob Zac Efron in 17 Again, out in August.

New behind-the-scenes shots of Zac Efron and Michelle Trachtenberg in their new movie 17 Again will be featured in the April 2008 issue of teen mag J-14.

In the mag, actress pal Miley Cyrus said, “Zac Efron and I have become best friends, and all these girls are like ‘you’re so lucky, he’s so hot’. …I give him the hardest time about it. He’s like my brother.”

Producer Adam Shankman recently chatted up 17 Again: “This is about a man whose life has not gone the way he wants it to, so he wishes he was 17 again and he gets his wish. And then he’s realizing it’s nothing like he thought it would be. So it’s a reverse Big. It has as many funny scenes in it as it does heartfelt scenes.”

17 Again opens this summer — August 2008!

Kelly Ripa drops by The Late Show with David Letterman on Monday night in a royal blue silk dress by Graham & Spencer. Michelle Trachtenberg paired the same stunningly lady-like number with Miu Miu flats to the 2007 Alvin Valley Fashion Show in L.A. in March.

Who wears it best: KELLY RIPA or MICHELLE TRACHTENBERG?

Can Tara Reid keep up a prim and proper rep from here on out? Tara, 31, seems to have turned over a new lead and is staying that way, looking pretty darn classy at the premiere of MGM’s Black Christmas held at the Mann Chinese Theatre last night in Hollywood, Calif. But will Hollywood ever take her seriously?

Black Christmas opens on Christmas, Dec. 25 in theaters nationwide and stars Michelle Trachtenberg (pictured below). It tells the story of a group of sorority sisters that find themselves being harassed by a stranger with menacing phone-calls and begin to die, one by one on Christmas Break. A remake of the 1974 horror movie, Black Christmas.

Celebrities came out in droves last night for Justin Timberlake and his BFF Trace Ayala’s first runway show for their clothing line William Rast. Who sat front row? Patrick Dempsey (aka Dr. McDreamy), Paris Hilton, Nick Carter, Michelle Trachtenberg, and fellow former N’Sync-ers JC Chasez and Lance Bass (inset with boyfriend Reichen Lehmkuhl). More pictures in the gallery of all the celebs!

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• American Idol Katherine McPhee is being considered for the big screen version of Wonder Woman. Kat will appear on the cover of Shape magazine wearing a bathing suit as a sort of try-out for the role.
• Gwen Stafani is said to be "boycotting" the MTV Video Music Awards because of the shoddy treatment she got last year. She was nominated for several awards in 2005 and every one, except best dressed, went to Kelly Clarkson. Gwen felt set up when she realized the camera was on her every time Kelly won.  Umm, I wouldn’t do that Gwen.  Just look at Janet Jackson now… (Her new music video got zero coverage.)
• After a cameo on CSI, Kevin Federline nabbed a role on Entourage playing a celebrity’s deadbeat husband.  "He is really keen — and not insulted when he was asked to play the part."