Swedish has du (singular) and ni (plural). Prior to the "du-reform" in the late 1960's however, ni was also used as a formal singular form, but for the last 40 or so years the formal form has been dropped, leaving ni for plural only. In fact many older people would find it mildly insulting to be addressed singular ni today.
Of course there are a few exceptions, most notably when addressing members of the royal family. If you ever get to speak with one of them, the correct way is to address them by title, in third person ("would her majesty like fries with that?").
Around 8 or 10 years ago I started to notice a trend, particularly among younger people (in their late teens maybe), typically working in shops and cafes, who started once again to use singular ni with customers, but I haven't lived in Sweden for several years now and don't know if that's continued. I hope not!
I read in an old grammar book that ni was actually a fairly late construct and that in Swedish history, there was another extremely archaic word ('I'?) which is directly related to U in Dutch and You in English. In fact, Wikipedia supports this "ni is derived from an older pronoun I, 'ye', for which verbs were always conjugated with the ending -en. I became ni when this conjugation was dropped; thus the n was moved from the end of the verb to the beginning of the pronoun."
I think this a modern misconception of the word ni. Prior to the "du-reform" it was used when talking to subordinates. The polite formal way was to use titles and last names.
The new use of ni as a polite pronoun is probably inspired by the rest of europe, as we're trying to be fashionably international.
Could this be dialectal? Because I hear "ni" now and then, and I've also found myself using it a couple of times -- not through intent, but rather automatically. Maybe it's the "new trend" you're talking about.
Maybe it's a central Stockholm thing, I don't really know. And I certainly didn't mean to imply that it was common, I only noticed it now and then and vaguely recall some mention of it on P1 Språket or in the newspaper, that it seemed to be making a return.
I've only ever known du, would never use ni except for a group, and feel odd being called ni myself. Not insulted mind you, closest I can describe it is like when a small child refers to you as "that man" but you're only 16 or 17. :)
Of course there are a few exceptions, most notably when addressing members of the royal family. If you ever get to speak with one of them, the correct way is to address them by title, in third person ("would her majesty like fries with that?").
Around 8 or 10 years ago I started to notice a trend, particularly among younger people (in their late teens maybe), typically working in shops and cafes, who started once again to use singular ni with customers, but I haven't lived in Sweden for several years now and don't know if that's continued. I hope not!