I've been to Zoorasia, about six years ago. It opened in 1999 and is expanding in stages. They still had golden snub nosed monkeys then. I remember seeing Tibetan macaques and golden takin for the first time. It's quite cool with mostly nice enclosures arranged geographically in attractive grounds.
That link definitely shows proboscis monkeys at Zoorasia.
From what I can gather from this blog and a few searches, the monkeys are from Surabaya Zoo and it looks like they actually arrived in March 2009 and were held under strict extended quarantine until June when they went on display with a welcome ceremony with the Indonesian ambassador where the monkeys' names were announced.
The enclosure is next to the Tibetan Macaques which I think is the former golden snub-nosed monkey enclosure. Interestingly this is the central Asian Highland area of the zoo rather than the Asian Tropical Forest zone where I thought they would be alongside the other langurs including douc langurs.
Zoorasia Map
By the way, the blog title is 'Tengu Blog'. Tengu is a kind of deity/monster/mythical creature which is usually portrayed with an enormous nose. Proboscis monkey in Japanese is tenguzaru or 'Tengu monkey'.
Edit: found this press release:
YOKOHAMA, JAPAN – Wednesday, May 20, 2009 – Proboscis Monkey (two males and three females) reared in Indonesia’s “Surabaya Zoo” have recently been brought in to join Yokohama Zoological Gardens “ZOORASIA”.
This entering was made possible following a mutual agreement made in 2007 to exchange breeding technologies for the Proboscis Monkey between the Botanical and Zoological Society of Surabaya and “ZOORASIA” under the Republic of Indonesia government approval. Hereafter, both zoos will cooperate with the breeding of Proboscis Monkey.
“ZOORASIA” becomes Japan’s only proboscis monkey rearing institution. The City of Yokohama and the Republic of Indonesia have also been jointly working for some time now in order to release “Rothschild's Mynah”, a bird endemic to Bali, into the wild.
The City of Yokohama places their initiative of breeding and exhibition of the precious animals as part of the international contribution activities, and expects that these activities will provide an opportunity for people of Yokohama to start thinking about environmental conservation issues on a global scale. Proboscis Monkey will be opened to the public on Sunday, June 7, when an official public opening ceremony will be held, inviting the Republic of Indonesia Ambassador to Japan and relevant officials from Indonesia.