As the main author of the Norwegian Naming Conventions page, I have to admit that there's a lot more that can be said about the sen/datter vs. son/dotter in Norway. The division has its roots in the 19th century national culture movement, with the intention of establishing or reviving a spesific Norwegian language and naming culture, in opposition to the previous 400 years of Danish rule and massive cultural influence. Thus, the son/dotter preference can be seen in conjunction with the "nynorsk" culture, which had and still has its strongholds in the western counties Hordaland, Sogn og Fjordane, and Møre og Romsdal, as well as in the interior parts of Oppland, Buskerud, and Telemark. About one quarter of the population of Norway today have nynorsk as their preferred language form.
It's worth noting, however, that the patronymic name forms which have survived as surnames in Norway, are totally dominated by -sen endings, even in the "nynorsk" areas.