I don't mean to be a name-dropper, but: I've had breakfast with Jeff Goldblum, partied with David Cross, and gone grocery shopping with Kate Winslet. Granted, none of these people knew I was with them at the time. They just happened to be in the same time/space as me. So goes life in Los Angeles. Live there for long enough, and you'll have a collection of celebrity sightings as well.
(The Best One: I peed next to Marilyn Manson after a screening of Paranormal Activity 3, and don't think that didn't have a traumatic effect on the 14-year-old inside of me who thought Manson was, quite literally, an agent of the devil.)