The Korea Times carried out a study of maps from 45 countries to ascertain prevalence of usage of the toponyms "East Sea" (preferred by Koreans) and "Sea of Japan" (the usage most prevalent worldwide). It found that 26% of maps it surveyed use both toponyms in this exact form: "Sea of Japan (East Sea)." An additional 3% use only the designation East Sea. Thus, 29% of the maps surveyed use the toponym East Sea in one way or the other.
Since this campaign by Korea to change the cartographic toponyming began only recently, in 2002, I'd say that a 29% usage rate internationally is fairly significant. Korea's campaign has been successful, but not ultimately satisfying to Koreans, as manifest on various Internet forums.
Furthermore, my fairly new (maybe 2007) National Geographic large wall map in my classroom uses both designations in the parenthetical form above. Also, my Prentice Hall high-school world geography textbook (World Geography: Building a Global Perspective, 2005) uses East Sea, in the parenthetical form above, in the four maps which appear in the chapter covering Japan and the Koreas. However, the textbook does not use the two toponyms elsewhere, e.g. in the atlas section; but, perhaps it will update all its maps in later editions.
The practice of using local toponyms, which would partly assuage nationalist sentiments, can be employed by Google maps because they can show local usage depending on where the person and his/her Internet server are located. Thus, Koreans, in Korea, accessing the East Asian region in Google maps would see the toponym East Sea. Not so easily done for printed maps and atlases.
This brings up a serious cartographic problem: Should the Korea Strait also be renamed "Korea Strait (Japan Strait)"? Should the Straits of Florida be renamed "Straits of Florida (Straits of Cuba /Straits of Bahamas)"? Should the Arabian Sea be parenthetically labelled Indian Sea or Pakistani Sea or both? I've heard that in Indonesia the ocean to its west is named the Indonesian Ocean. How about the U.S. renaming the Gulf of Mexico the Sea of Mississippi?
Perhaps if rampant nationalism slowed down there would not be such interest in this cartographic dilemma. Not much hope for that.