As a usability engineer currently watching someone perform a usability test involving filling out forms (okay, maybe not the world's most attentive usability engineer :)) I want to plea/remind/admonish form designers to make absolutely as little as possible required. Carefully weigh the expected benefit of more complete data (who will use it?) against user frustration and increased failure rate. If people like you, they will fill out everything they can, required or not. If they don't, they'll lie on the required fields or just leave, depending on how badly they want what you have.