Although I like a nice bark, I rarely have time to really create one. I generally smoke for 5 hours and put it on heavy - I am now using two AMNTSs with hickory at the same time for pork butts. Then I foil and start checking temp 2 - 2 1/2 hours later. The 8-9 pounders I usually buy from Sam's usually take about 9 - 10 hours. I like to pull mine when all of my temps are at least at 195 as the connective tissue is just broken down but not it has not started to cook out. Most cook theirs over 200. It's just personal preference in the texture of the meat, but make sure that the whole roast is at least over 190 or it won't "pull". Also, use the foiling juice as a finishing sauce.
Although I don't get a true bark, I get a lot of smoke build up on the outside (like what ChefJimmyJ was talking about). Some of this seems to penetrate the meat when it's foiled. However, I do a few butts at a time, pull them, and then vacuum pack enough for a meal and stick them in the freezer for quick meals. With the "bark" all mixed up once it's pulled, the smoke flavor seems to go all though the meat as it sits all mixed up together. Just take the bag out of the freezer and drop it in boiling water for a few minutes and BAM!! thirty minute pulled pork!
This is just my way of doing it. Play around and you'll find what you like. This site is full of ideas. One thing I like to do (but Mrs doesn't, so I don't do it often) is to slice up a sweet onion and layer it all over the butt when I foil it.