Whatever some would like to call it, since every font has dashes
of different (relative) lengths* it really becomes more a matter
of what looks right.

* Not to mention sidebearing strategy.

hhp

You need an em dash, *not* a hyphen or an en dash.

One might also reasonably use a quotation dash (“horizontal bar,” U+2015). But not all fonts have those. In any case, they are often the same width as the em dash. (Despite what you might guess from the name, these are primarily used to set off the actual quoted text instead of an opening quote mark, in languages where that is the typographic convention.)

Definitely an em dash. Or nothing.

I put a space after the dash, but if your "em dash" has space built in around it, maybe you wouldn't do that. Some people use no space.

I third em.

The quotation dash is usually an En dash, at least here in Sweden. But yeah, probably Em dash here.

Thanks so much, everyone! Em dash, it is.

And don’t forget small caps for the name.
Thus with the main text in roman, the quote in italic and the attribution in small caps, the three main lower case styles are used.

Jeez, guys, it is a matter of style. Now you've given Ali the notion he's learned a "truth."

How about "in the States (and probably Canada, since Nick agreed), the em dash would be the usual convention."

Britain as well. Beyond that, I can't say.

Many typographic “truths” are language-dependent or culturally-dependent. This would not be unusual in that regard....

And the dash, of any sort, is not required. There is generally no typographic nor editorial necessity. It is all a matter of style and convention, as Charles notes.

There are plenty of designers and editors who dismiss the dash in this circumstance.

I agree that an em dash looks best, but does anyone know if this is specified in any of the major style guides (Chicago Manual of Style, etc)?

JamesM, I checked in the Chicago Manual of Style, and couldn't find a preference there.

This Chicago Manual page doesn't specify the dash but shows an em dash in the example (as well as an undashed example).

A book of quotations I have (two columns per page) sets the attribution flush right, no preceding dash.

My Webster's dictionary has a manual of style which they based on the USGPO style guide. It recommends an em dash before the attribution.

http://typophile.com/node/46490

Personally I like the tilde with the curved-in ends, the ones that look slightly like a skinny sideways S. I suppose those this would only work with a font that matches that style somewhat, probably a serif. Dont think it would look quite right next to Futura xtra bold or the like.

What Nick Shinn and Kent Lew said. No dash should be required in quotes. Plus, you shouldn't use the quotation marks either. Just set the quote in roman and the author's name in small caps.

Ryan, I often use a tilde too, but I realize it's eccentric and less formal looking.

I favour the em dash. Peter Sheerin has a good article on A List Apart.
http://www.alistapart.com/articles/emen/