Today, I read this fascinating article which ran a few weeks ago over at msnbc.com.
Consumers of some of the newer energy-efficient front-loading washers are experiencing mold in the rubber gasket which seals the door.
In a top-loading washer, any condensation usually evaporates. In a front-loading washer, however, water might collect on the bottom of the rubber gasket, which could lead to mold.
I highly suggest you read the article if you own a front-loading washer. It does provide tips on how to prevent this problem.