Epiphyllum oxypetalum, Cactaceae.
Also known as Dutchman's pipe cactus] or queen of the night. It usually blooms only at night (about 8 pm till might night). But on this lucky day (2012.9.19), it started blooming around 5 pm when it was let to bloom outside and one bee started foraging on it. I was in China so my wife Joyce took these photos.
On Aug 23 (first two photos), it was blooming during the day also, but no bees were seen.
Chinese name: tanhua昙花. Tanhua yixian is a chinese saying meaning that good things last only a short time.