We took the car in for a service recently, and the service writer pointed out that the rear main oil seal was leaking. This was a rather abrupt change, and it's really leaking (drive 20 miles or so, park, get 4-6 inch puddle on the driveway overnight).

The parts guy at the dealer says this is a known problem. I'm wondering whether I should be concerned about causes, like excess crankcase pressure, or whether it's true that these go more frequently than average.