The cases I can find that are most on point is Walkovszky v. Carlton, where a claim of underinsurance was directly at issue (unsuccessful because the insurance that was carried met an explicit statutory requirement) and Autrey v. 22 Texas Services Inc. (on a summary judgement motion finding there are facts in dispute, so not squarely ruled on).

A couple of relevant papers that discuss insurance throughout:

http://scholarship.kentlaw.iit.edu/cgi/viewcontent.cgi?artic...

http://www.law.emory.edu/fileadmin/journals/elj/56/5/Millon....