Insect Egg Case, and if so, What Insect?
A local graduate student relayed the answer to me.  It is an ootheca of a praying mantis.  There is some additional background on the egg life history here.  Wikipedia reports there are three mantid species.  The upper-left picture of the ootheca here is essentially what I observed, so we may be looking at the genus Stagmomantis.  There are some other great pics at that last site.