I have been collecting monthly PostgreSQL regression test code coverage reports at <http://developer.postgresql.org/~petere/coverage/>. So if you are wondering what this thing is but haven't had the courage to try it out yourself, there is your chance. (Hmm, buildfarm integration could be nice, someday.)
We have had a line coverage rate of about 66% steadily for a few months now (well, it's feature freeze). The lcov tool labels that as "green" (=good). The new version of lcov, which I have in use as of the April report, also reports function coverage, where we have about 73%, which lcov labels as "red" (=bad).
For the next release cycle, I have two goals in this area: First, expand the test coverage reporting to the entire source tree, not only the backend. And second, improve the test coverage of various neglected areas. There is reduced coverage, for example, in the areas of non-btree indexes, vacuuming, recovery, GEQO; and once we analyze other parts of the source tree, we will probably find gaping holes there.