I fall on the side of the teachers in this debate. Besides the practical considerations (prove I didn't write this lesson plan at home last weekend), I can find no compelling reason for school districts to lay claim to my intellectual efforts.
Update, 11/20/09: My contract doesn't address the issue at all, but here are two statements I found in district policies:
Use of district computers, computer networks, and Internet services does not create any expectation of privacy. Work produced by any employee on a district computer shall be the property of the district.So as long as I don't use district equipment to create my lesson plans, or use district computers to advertise that they are for sale, I'm safe.
District computers may not be used for personal commercial purposes, including offering or providing goods or services.
Now I just have to be creative enough to create lesson plans that people would want to buy.