Thanks so much for all your help in identifying this plant. Marguerite was the first to respond, with the correct identifying information even! Moonflowers apparently bloom at night (this one did) and can remain in bloom throughout the next day (this one did). It is all shriveled up today however, but good news is that there is another large bud that should open in a few days.
Moonflowers are a relative of the Morning Glory, and apparently can be invasive as are the smaller morning glory. (My DH fights the smaller ones in his garden every year). We'll worry about too many Moonflowers some other time...right now I am really enjoying this plant.
It is an annual, and well self-seed under the right conditions. We will probably collect the seed pods and try again next year however. I think they deserve prime real estate next year - not the obscure corner they called home this year.
It does seem similar to a Hibiscus or a Holly Hock, but foliage is not the same. I am convinced that it is indeed a MoonFlower. I'll be out under the moon watching for the other bloom to open during the next few nights.
Again - thanks so much -- Mystery solved! And I am now more informed as to how to care for it.
Husbands - you have to watch them all the time!!!