imho

Maybe broader context would shed more light, but my reading of the source text is that what it calls in the first place, using "proper" French, the "commissaire d'exposition" is simple what it goes on to call, franglais style, "le curator" but that these are as one and the same.

I read the source text as presenting some of the issues surrounding the custom of the curator setting a theme for an exhibition. It starts by saying it's normal that this occurs, then says, referring to the same gesture, that, "besides the danger (of this practice ie the curator setting a theme) of the works simply illustrating the curator's thesis, artists often..."

Wiki defines a "commissaire d'exposition" as "l’organisateur d’une exposition artistique: il sélectionne les œuvres et les artistes exposés, fait les choix relatifs au catalogue et y rédige, souvent, des textes. Il est donc l'auteur de l'exposition..."
http://fr.wikipedia.org/wiki/Commissaire_d'exposition

I don't know what that is if not a curator!

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Note added at 17 hrs (2009-05-29 06:37:29 GMT)
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As Diane says, "exhibition curator" would be the full title, to distinguish this kind of curator from the "conservateur" kind.