Are you really sure about? If you read the below text it actually even says "should" rather than "must". However this page also says that passport is the only way to provide identiy when travelling to other Schengen countries which seems to be out-of-date or wrong after the national ID card has been introduced.
The Swedish Police website says:
"Vid resor inom Norden krävs inte pass för svensk medborgare. Du bör dock ha med en giltig legitimation för att styrka din identitet, bland annat om du tar in på hotell, campingplats eller liknande."
"When travelling within the Nordic Countries passport is not required for Swedish citizens. You should however have a vaild id to prove your identity, when check in at hotels, camping site and similar."
http://www.polisen.se/inter/nodeid=3...eversion=1.jsp
In order to take advantage of passport-free travel between the Nordic countries, you must be able to prove that you are a Nordic citizen. The only way to prove that is by showing ID that shows your nationality. New national IDs and passports are enough, a Swedish driver's license isn't.
The person/authority/hotel/etc. may have good reasons to believe that you are a Nordic citizen based on languages you speak (or your ability to stand 15+ minutes in a hot sauna, which would make you Finnish
), but the only sure way to prove it is by showing the proper ID.