The only thing to really watch for is if the duck is molting and getting soaked/icy or he or she is having trouble with waterproofing. We've had one duck (Martha) we've been watching closely because she was getting soaked with single digit F temps. Unlike our other ducks, the water was not rolling off her back and she was getting super icy. Yesterday morning she was a mess, but she went into the duckhouse and preened and preened and she looked great by the evening (I've posted some pictures and a video in the thread I started about her feathers).
I would NOT bring a duck into the house if it's very cold outside unless you plan to keep it in the house or there is really a crisis. I would be very worried about significant temperature swings.
We've been so thankful with all the help we have received here at BYC, and I'd encourage you to keep asking questions, post pictures and videos and share your experiences.