Single Kristin Cavallari Celebrates 21st Birthday in Vegas

Nick Zano is apparently out of the picture.

While Tom Brady and Tony Romo prepare for NFL playoff games, a bored Matt Leinart apparently passes his time partying in Las Vegas.

Both of these revelations and many others stemmed from Saturday night in Sin City, where Kristin Cavallari celebrated her 21st birthday.

The former Laguna Beach star wouldn't divulge whether she will be joining MTV's The Hills, only saying that "it's a great show."

But the SoCal cutie certainly partied like The Hills cast does as she got down to Soulja Boy on the dance floor and hung with friends.

"My first official drink was Grey Goose and soda," she said. "It's really exciting. I can go to the clubs and not worry about anything."

Kristin Cavallari was joined by fellow Laguna Beach alumna and good friend Alex Murrel, spent the night chatting up Matt Leinart, the USC grad and current Arizona Cardinals quarterback who's also a reputed former flame of hers.

Following her dinner at Company American Bistro, Kristin Cavallari's highlight was the sparkling birthday cake that was brought out for her.

Saturday night capped the weekend-long celebration for the infamous and sexy high school rival of Lauren Conrad. As soon as the clocks struck midnight on Saturday, a newly single Cavallari - who recently broke up with longtime beau Nick Zano - headed to Pure Nightclub to take in the sights.

As for her love life? Kristin Cavallari wouldn't reveal details on the boy(s) she's seeing, only saying that she is dating "and having fun."