Resurrected old thread. I'm still awake, so why not comment.
I cut most of the fat cap off of a pork butt. I leave just a thin layer. Too much fat just keeps rub and smoke from penetrating the meat, and smoke has a hard time getting deep into a butt even with a 12-hour cook.
I go fat cap up, for some "self-basting," but honestly can't say with certainty that it matters. It is just how I do it.
I never foil a pork butt during cooking. IMO, pork butt is the easiest, most forgiving chunk of meat to BBQ. Why make it harder than it needs to be?
Pork butt is also a great item to pre-cook for a party across town. I wrap the finished butt in foil and a towel, and stuff it in a cheap cooler, and like Craig said, it is still too hot to handle after a few hours.
My neighbors listen to good music whether they like it or not.