Last Christmas, I was on a multi-hop flight back to SF from the east coast. We were going to be landing in Vegas, and I was talking to a flight attendant. She mentions that conditions in Vegas were a little crazy, as the runway doesn't drain properly and crosswinds are common.
As we're preparing to land, the captain gives the usual speech. It's going to be a little rocky on the landing; there's a sleet storm, etc.
We're making our approach, and the flight attendant gets on the PA. "Someone's cell phone is on. Turn it OFF NOW." A minute later: "Someone's cell phone is still on. I need EVERYONE to doublecheck their phones, and get that phone switched off!"
We ended up diverting to Phoenix, because the pilot did not feel comfortable making an instruments-only landing. His announcement was the only time I've ever heard a pilot sound audibly rattled on the PA.