This small finch-like bird is a Red-billed Leiothrix, a southeast Asian bird that has now established itself on the island of Hawaii. They were first introduced to Kauai in 1918, and later were brought to the other islands as well, where they established substantial populations.
Since that time, they've actually been added to the CITES Appendix II because of habitat destruction and capture for the caged bird market. However, they still maintain healthy populations in their introduced home, except - ironically - Kauai, where they have disappeared.
The two at the Philadelphia Zoo at this time were donated to the zoo by a collector, and were introduced last summer.
Update, March 2006: I was contacted by the Philadelphia Zoo the other day, and they want to use some of Shari and my pictures in their publications, and possibly at the zoo. They've already picked out 25 shots that they like, including this one. I'm pretty psyched to see that my pictures were that well-received. Of course, we're both donating limited rights - I'll let you know more when I do!