Article about the group that writes the space shuttle software, sort of relevant?: http://www.fastcompany.com/magazine/06/writestuff.html
A full run of our test suite literally takes months on a cluster of hundreds of CPUs (obviously, there are also faster versions of the tests which are run frequently). While I have a long list of additional test coverage that I would like to add, what we test is much closer to "all functionality" than it is to "a few critical things".
I'd say that a lot of good responses would have been in between those two.
I think it's safe to assume that anyone who selected "All functionality" actually means "Most functionality". Also I think we can assume that a good proportion of people who selected "A few critical" would belong in the "Most" bucket.
When I saw my codebase has 'all' functionality tested, I mean we don't commit code without tests included too. I think that's a pretty reasonable definition.
double sin(double x) { return x; }
However, even if the sin function is already tested elsewhere, you will still need further testing to ensure that you are calling it correctly (e.g. not confusing degrees and radians).
EDIT: Yes, I read it wrong - clearly need coffee...
That said... that kind of coverage isn't quite as useless as it might seem. If your tests do execute every line, even in a completely contrived way, you will catch a lot if you change your code. You just tend to catch more of the "wrong number of arguments passed to a method" kind of error than "you are allowing the autopilot to try to land the plane 100 feet below the runway" kind of error ;)
However, if you consider functions on the code level (e.g. Java methods) then organisations with 100% coverage will be thin on the ground. If you go further and consider line coverage, almost nobody will have 100% coverage.
A common problem with organisations claiming to test all functions is that they will only test the happy path - there will be few tests for things like unexpected or illegal input etc.