I have been following the discussion on time-based art with much interest, and now Charlie has brought up 'Skypoems' and 'Tilted Arc' I find myself thinking about the topic in the context of the inseparability of the phenomenology of the art experience and the ontology of the art object.
It seems to me that both works invited the viewer to engage with the time of a decision as to whether/what work is art. In the same way I understand Danto's reference in 'Transfiguration of the Commonplace' to Serra's 'Corner-Piece' in terms of drawing the viewer's attention to the time involved in deciding whether/what will be responded to as art.
I've always been particularly curious about a remark one commentator made regarding one of the judgements made deciding on the question of whether 'Tilted Arc' could be removed from its location. It was suggested the judge treated the work as if it were like any other object, such as a toilet. 'Tilted Arc' as a readymade?
In terms of the time of the decision I have in mind Derrida's reference to "a thinking of urgency" that is involved when the need for a decision is opened by art ('Echographies of Television'). Although the answer may come very quickly given its institutionalisation it seems to me that 'Fountain' still invites a decision as to whether Duchamp was being serious, joking or perhaps as Louise Norton suggested, both.
I find Charlie's comment about anthropological museums particularly interesting because of the approaches adopted in (online) museums dealing with the intangible culture of First People. My impression is that in such contexts viewer interactivity, so highly valued elswhere, is heavily qualified in favour of privileging the voice of the indigenous culture.