[Corrected 21 Sep 2010.] Here's another odd plural. The plural of genius is... geniuses. I originally thought that since the word had Latin roots, it would take the common Latin plural of genii. But it's one of those words whose evolution has led the use of genii (pronounced gee-nee-i) to be restricted solely as the plural of mythical spirits.
Incidentally, the word etymology of genius is fascinating. It used to refer to the attendant spirit present at one's birth, then it evolved to mean a person's characteristic or disposition and that led to the sense of a person's natural ability. And that evolved so that it now referred to someone's exceptional natural ability. So there's the link between the spirit and brain power. See OxfordDictionaries.com.