The George Mason University 2005-06 University Catalog states:
Students are expected to attend the class periods of the courses for which they register. In-class participation is important to the individual student and to the class as a whole. Because class participation may be a factor in grading, instructors may use absence, tardiness, or early departure as de facto evidence of non-participation. Students who miss an exam with an acceptable excuse may be penalized according to the individual instructor's grading policy, as stated in the course syllabus. (http://www.gmu.edu/catalog/apolicies/index.html#Anchor38)
As you develop your course, be sure in your attendance policy to equate a lack of attendance to a lack of participation. The policy should be explicitly stated in the syllabus.