It did not come as a big surprise when the news arrived a few days ago (see Telegraph.co.uk 14 February 2011) that Prince William of Wales had asked his brother Prince Henry ("Harry") to act as his best man at the royal wedding on 29 April 2011. Kate Middleton's maid of honour will be her sister Philippa.
Also in Norway it has been the tradition that close family members have acted as best man, as we have seen at the 3 Crown Prince weddings that have taken place since Norway chose a new royal dynasty in 1905. It is no big news that the then Crown Prince Olav's first cousin - the then Duke of York, later King George VI - acted as his best man when he married Princess Märtha of Sweden in 1929. The Duke of York did his duty in Vår Frelsers kirke (Our Saviour's Church, today Oslo domkirke/Oslo Cathedral) on 21 March 1929. But formally it was Crown Prince Olav's aide-de-camp, Captain Nikolai Ramm Østgaard, who was the Crown Prince' "forlover" ("sponsor", as the banns records 1929 states.
The marriage act 1918 (as the present 1991 act also does) required that "Each of the parties to the marriage shall provide a sponsor who shall solemnly declare that he or she knows the said party well, and shall inform whether the said party has previously contracted a marriage and whether the parties to the marriage are related to each other" (as described otherwise in the said act). Each sponsor had to sign a declaration in this regard. Of course it was more practical that the aide-de-camp signed the papers. Just a formality, of course, and the Duke of York should still be referred to as Crown Prince Olav's best man in the history books. But Captain Østgaard's role is perhaps worth a footnote, or at least a blog article? :-)
Princess Märtha's "forlover" (sponsor) was her father Prince Carl's Kammarherre (Chamberlain) Friherre (Baron) Nils Fredrik Stiernstedt.
When then Crown Prince Harald, today King Harald V, married Sonja Haraldsen on 29 August 1968, he had chosen his first cousin Count Flemming of Rosenborg (formerly Prince Flemming of Denmark) as his best man. But the person who signed the sponsor declaration was his aide-de-camp, Stein Evju, as the banns records 1968 shows (no. 21). Ilmi Riddervold was Sonja Haraldsen's maid of honour, and she also signed the declaration herself. Much easier when you live in the city.
When Crown Prince Haakon married Mette-Marit Tjessem Høiby in 2001, his best man was his third cousin, Crown Prince Frederik of Denmark. Reading the banns in church was not compulsory by law then, and the "forlovererklæring" (sponsor declaration) is not (yet) a public record, so at present we do not know if Crown Prince Frederik actually signed the declaration himself, or if someone at the court did in his place.
The arrangements described above was not unusual for commoners as well. When Queen Sonja's uncle Rolf Ulrichsen (1892-1967) married Isborg Dahl in 1926, his friend Viggo Seaward acted as his best man (see Skien parish register copy no. 10 Marriage records 1926, p. 288 (no. 48), while his brother-in-law (?) Halvor Dahl had signed the declaration (banns records 1926 (no. 39)). The couple later divorced, by the way, and Rolf later remarried (a woman named Sonja!).
Banns records for the marriages of Princess Ragnhild and Erling Lorentzen in 1953 and Princess Astrid and Johan Martin Ferner in 1961 - both couples got married in Asker - don't seem to be available at The Digital Archives.