Hi Jeremy,
The Singapore Zoo houses orang utans in 3 different types of display:
1. The "Island" as you mentioned. Live trees were recently planted on the island, but all placed in the centre, away from the island's perimeter. Recently hot wire was placed on the trees not to prevent escape but to let the trees mature as the orangs were destroying the new shoots. All the trees planted on the island are ficus species. As for the moat, the zoo has never had any problems and in fact a couple of the orangs actually enjoy a dip in the water on hot afternoons! The design of the moat edge is important, with a gradual slope leading to deeper water. This reduces the chance of drowning.
The island exhibit has been at the zoo for over 30 years.
2. "Free-ranging on live trees". 2 areas have been set up, each with about 6, 7 large trees (over 15 metres tall). Hot wire is placed on the trunks to prevent orangs from descending to the ground. Trees in the surrounding area are pruned to prevent the orangs from escaping via connecting branches. Since it was set up more than a year ago, there has been 1 "escape" (the hot wire failed. But the orangs were all raised with free contact so the keepers simply strolled over and took the orangs back to their dens by hand) and no falls. However, only the younger females and adolescents are allowed in these free-ranging areas as larger, older animals are more prone to falling.
3. "Enclosed cage". This is a new enclosure (opened in July 2007) to house a breeding pair of Sumatrans, with a glass viewing wall and fully enclosed mesh. This enclosure is similarly heavily planted.
You can have a look at my gallery page. i've posted photos of all the 3 types of enclosures.