I wonder if it could be this local version of Morning Glory? Ipomoea cordatotriloba http://www.sbs.utexas.edu/bio406d/images/pics/cnv/ipomoea_cordatotriloba.htm http://davesgarden.com/guides/pf/go/53063/ HTH Chris
It does look similar. It's a bit back in the weeds, so I haven't gotten up next to it recently (fire ants.) Now I'll have to either go early in the morning, or brave the bugs and look at it more carefully for closed flowers. There weren't any when I took that picture, but there could be by now... Thanks.