Great call by everyone below. Regular rum will not light on fire. Has to be a minimum proof of 150 for it to light. Tracy, try it with Corona, then it tastes light Dr Pepper. Light beer tastes more like Root Beer for some reason.
I do not claim to be a mixologist on top of which this is my first post on this website. I will however tell you the manner that I learned to make the flaming dr. pepper is dramatically different from what's presented here. Before judging I suggest you give it a try.
Draw a beer into a pint glass, chilled or not, set the beer aside. In a double shot glass pour a single shot of amaretto, top with a single shot of bacardi 151. Touch a lighter to the resulting double shot to set it aflame. Having done so drop the flaming amaretto/bacardi mixture into the pint glass of beer that you have previously set aside, drink immediately as if you are drinking an Irish Car Bomb. You will find that it will produce a dramatically different result than the method suggested by liqour.com and that suggested by the previous commenter which will taste more like dr. pepper than root beer. Too much amaretto in both suggested examples.
Now the way I learned to make a Flaming Dr. Pepper, (I have worked as a bar tender) is .75oz. Root beer schnaps, .25oz. Amaretto, and a splash of 151 dark rum on top. Carefully light shot, drop into beer and Slam it back and you will think you are drinking a root beer float! No idea why they call it a Dr. Pepper when the darn thing tastes more like root beer but hell they are damn good, but they will catch up to you with a quickness!