In the South American house at Emmen zoo - the Netherlands there's a quite large ( several square meters ) open-topped poison dart frog enclosure. To protect the frogs for the free-ranging birds and monkeys - or otherwise ? - the top is however covered with netting.
Tulsa Zoo has an open-topped exhibit for poison dart frogs: http://www.zoochat.com/1247/tropical-american-rain-forest-barrier-free-170260/
I just remember the walls of the Tulsa exhibit to be quite smooth and steep, although I could speculate that there was another barrier impeding the progress of the frogs. Maybe the walls were too slick for the frogs to get a grip...
From my experience of keeping PDF's privately, though not having seen the exhibits mentioned, I would think they have a lip of some sort around the top. They can climb well on glass by pure friction even as adults, and froglets are even better at it, I have had them jump out from the top of a 60cm tank as soon as I took the top off.
Not exactly a PDF exhibit, but Bristol Zoo has free ranging Trinidad Stream Frogs living around the waterfall pool in the middle of their reptile house. They regularly breed in water bowls put out for them. Unfortunately, they are the exact colour of the rocks so they are hard to spot unless a male is calling, but they seem to maintain themselves quite well.
Deep sea world, when I last visited, they had PDF in a large vivarium, it was even difficult to spot a PDF. It isn't open-topped though.
