100% code coverage only means all your code has been run, not that it has been tested. There is a huge difference.

I read a good paper called "How to misuse Code Coverage" a while ago that discusses exactly your point. It is available here as a PDF. Its worth a read if you haven't read it already.

Personally, I shoot for 95% and above and I am happy. As I described above, I try not to worry about coverage until the end, that way I am concentrating on testing my code and not covering it. And lets not forget too that unit-tests only test isolated chunks of code. Integration tests are just as important (if not more).