I recently watched something about this on Larry King...that often the celebs call the paps to let them know where they'll be! I'm sure that isn't true in a lot of cases, but I felt pretty silly not knowing that little fact. She didn't seem to try to lay low!
Paris does this all the time and Nicole Richie did too. That is pretty much how these two launched their careers. And Lilo used to do this all the time too. That is why you would always see them in the celeb weeklies. Britney allegedly was always in close contact with the paps during her meltdown ... and before she started dating Adnan. A lot of the celebs go to places where they know the paps will be. Why do you think they all go to lunch at The Ivy? It ain't for the $30 Cobb salad.
Like another poster said, Sienna isn't exactly known for acting abilities. She hasn't really had any roles of note. I think the only reason a lot of people know who she is because she is in the celeb weeklies.