I read a short where Clark finally gets around to telling her between the wedding and the reception. If I remember correctly, she faints, and Martha scolds him pretty heavily for not having told Lois beforehand.

"It is a remarkable dichotomy. In many ways, Clark is the most human of us all. Then...he shoots fire from the skies, and it is difficult not to think of him as a god. And how fortunate we all are that it does not occur to him." -Batman (in Superman/Batman #3 by Jeph Loeb)