Sigur Rós regularly does that, including all vocals on their 4th album "()": http://en.wikipedia.org/wiki/Vonlenska#Vonlenska

Try listening to old Cocteau Twins sometime. Similar effect, except Elizabeth Fraser is actually singing in English.

As an aside, Mike Patton apparently speaks Italian fluently, having been married to an Italian woman, and IIRC, he recently did an album of old Italian pop songs. Perhaps he and Celentano have sung some of the same tunes, which is kind of a funny thought.