Daox,
Looks like I will be installing some of it where the rafters meet the ceiling and some around the light fixtures in the attic. Also on the attic access door.
After reading the responses and the article that Exeric posted under "Spray Foam Experiences", it looks like it would be a waste of money to use it on the rafters.
Here is the article Exeric posted.
http://www.buildingscience.com/docum...-sealing-guide