It appears to me to be a Night Blooming Cereus:
We had one in our greenhouse in Botany. It rarely bloomed, and the bloom was wilted by mid-morning.
Night-blooming cereus is the common name referring to a large number of flowering ceroid cacti that bloom at night. The flowers are short lived, and some of these species, such as Selenicereus grandiflorus, bloom only once a year, for a single night. Wikipedia
Dave's garden lists it as "Dutchman's Pipe Cactus, Night blooming Cereus" Epiphyllum oxypetalum.