Seen in Jamaica, West Indies. 20-25 foot tree. Flowers looked like mimosa, 8 inch diameter. Bloomed every night, dropped the blooms at dawn.
Your description of a tree that blooms at night and flowers fall off in the morning sounds like Nyctanthes arbor-tristis: http://davesgarden.com/guides/pf/go/95610/ However, its flowers don't bear any resemblance to mimosa so maybe there's something else that also behaves that way.
I was on vacation, didn't have a camera. I asked locals (they always knew what plants were) but they were stumped.
Did you check the link I provided to see if that looks like what you saw?
Yes, ECRANE3, I looked. It's nothing like the flowers I saw. As I said, they had a diameter of about 8 inches, and were single, not in bunches. They looked just like mimosa, huge feathery things, spectacular! The leaves of the tree also looked like Mimosa.