I had pork loin last night in fact. It's a cut that is tender enough to cook relatively quickly; I'm not sure it will benefit from the low and slow treatment of a smoker but others here may disagree.
I put a light mustard slather on mine (just cheapo French's) and then coat it liberally with salt and pepper or your favorite rub. Then I put an empty cast iron skillet on the grill and let it get rocket hot for about 15 min or so. Sear/Brown the loin on all sides and remove the loin and skillet from the grill. Place a couple stems of rosemary under the loin if you have it. I then put (shhh...don't tell anybody) it, whispering now, in the oven at 350 and cook to 138F. Takes about an hour but it can vary of course.
Let it sit for 10 min on a cutting board while you make gravy from the stuff in the skillet.
Make sure the loin is on a cutting board with a drip channel around the edge, the deeper the better. Cut the loin into thin slices and pour the juice that collects in the channel into the skillet with the gravy. You'll probably have to do that more than once.
Eat, wash, repeat..