Update Aug 20, 2009: The U.S. Odonata Checklist Comittee have decided on a common name for this taxon: Red-mantled Skimmer.
I found this skimmer at Willow Pond #1, Santa Ana NWR, Hidalgo county, Texas on August 11, 2009; I sent the images to Dennis Paulson, and he has identified it as a male Libellula gaigei. This taxon is previously known from five Mexican states:- Veracruz, Sinaloa, Campeche, Quintana Roo, and Yucatan. It was described by Leonora Gloyd in 1938 from two specimens collected by others in the Yucatan prior to 1932. Dennis informs me that it is a rare or rarely encountered and poorly-known taxon - he has never seen one on his many trips to Mexico - and that these are almost certainly the first photos ever taken of it: