Frost in the attic is no different than anywhere else. It is caused by warm moist air coming into contact with cold surfaces and dropping its moisture. The problem in attics is primarily one of quantity; in one house frost accumulations of up to fifteen hundred pounds were discovered one cold February.
Warm air rising in the house pushes moisture up through cracks in the ceiling. Below freezing temperatures make ventilation, even power ventilation, ineffective as a drying-out mechanism. If there are no warm spells during the winter in the attic, the frost will simply pile up all winter long.
It is too expensive to warm up the attic (that's why we put all that insulation up there) and ventilation can only help during natural warm spells and the spring thaw. The primary defense against frost in the attic is a well-sealed house. (For the complete story on air sealing, go to the Nuts & Bolts section and check out Air Sealing.) In some cases this can only be achieved by sealing the upper shell of the attic -- closing off all ventilation holes. (search keyword "attic" for the title "MYTH: THE MORE ATTIC VENTILATION IN WINTER THE BETTER." for cautions and details on this tactic.)