With a @font-face declaration like the following:
src: local('☺'), url('http://paulirish.com/tagesschrift.ttf') format("truetype");
... IE9 pp3 downloads both the eot file and the ttf.
The css3 font spec says this: "When a font is needed the user agent iterates over the set of references listed, using the first one it can successfully activate. "
While admittedly the syntax here breaks spec, it has been used to deal with IE<=8's shortcomingings with parsing a valid @font-face rule.
It is now recommended and used by leading webfonts sites: Font Squirrel, Typotheque, Fontdeck, and WebINK.
Moreover, using totally compliant syntax still incurs the double resource download:
(I did not test with a data URI as the first src value, but I would expect a similar behavior there)
Needlessly downloading both resources is inefficient, non-compliant, and slows down the user experience.