What Was Topless Nicole Richie Celebrating in Cabo?

Paris Hilton was detained in Japan and Lindsay Lohan has a warrant out for her arrest, but life's a beach for their former BFF Nicole Richie.

The hot mama, who turns 30 today, was celebrating South of the Border in a bikini bottom and not much else. She was joined by famous friends Christina Aguilera and Sam Ronson in Cabo, and she may have been celebrating more than just her birthday...

While having dinner at a local restaurant, Nic was spotted wearing a "bride-to-be" sash and necklace adorned with plastic penises.

Was this trip also a bachelorette party?

Nicole's rep didn't respond to our request for comment on what exactly she was celebrating, but either way, happy birthday!

Nice to see the former wild child is staying out of trouble but can still cut loose!