I worried over that just the first few times I cooked them, but I've never smoked a whole turkey, either.
Since just the breast is fairly small (about 7 pounds) and all white meat, I build me one big chimney of coals, and add green hickory (and nuts) and slices of apple when my smoke gets low and just keep at it till my fire peters out (about 4 hours). As soon as I take the hot meat off the smoker, I wrap it in a double layer of heavy foil, which to my mind ought to ensure the bird is completely cooked thru. And I always do it a day or two ahead of time, so I'm not tracking in and out of the house while I'm cooking the day guests arrive.
I'm sure the USDA and hardcore BBQ folks would disapprove of the method, but it's always cooked thru and always moist and flavorful, and frankly way easier than I let on when folks talk about how good it is. ;-) Works for me.