While picking no particular bones with Quizmonkey, I would, after consulting my copy of Grammar Tips state there's a difference between the way British and American's use these conventions. "The American rule is that double quotation marks are the standard form, and single quotation marks (what the British call "inverted commas") are normally used only to enclose a quotation within a quotation.
The British usually, but not always, reverse this order, using single quotation marks first, and then double quotation marks to enclose quotations within quotations."
Additionally, and in line with Quiz's example, "...And that brings us to the point about using double quotation marks for enclosing words or phrases that are not actual quotations but that are being set off for some other reason--i.e., words referred to as words."
"When a word is being defined or otherwise set apart as a word, it needs to be rendered in some way that will distinguish it from the rest of the text. For words presented as vocabulary terms, either italics or boldface can be used."