When I did my first pork butt a couple of years ago, my wife objected to regular tomato-based barbecue sauce because she grew up eating way too much of it. So I had to come up with something different. She LOVES mustard, so this made perfect sense. I decided to Google "mustard barbecue sauce" and the first recipe that came up was one from barbecue.about.com (and it's still the first hit on a Google search). I have to say it's a fantastic recipe, very simple, with a lot of vinegar and a spicy kick!
I dub it "Pork Paint".
1 cup prepared yellow mustard
1/2 cup balsamic vinegar
1/3 cup brown sugar
2 tablespoons butter (I skip this, makes it too creamy for my liking)
1 tablespoon Worcestershire sauce
1 tablespoon lemon juice
1 teaspoon cayenne
Mix all ingredients together and simmer over a low heat for 30 minutes.
The balsamic is very powerful, but I love the stuff!