Mogly's and Animatorjones are great so far!
Tystarr, yes that bg is loopable. Make your buildings one big movieclip and email it to me in an fla if you don't get the following instructions, so I could do it for you, easily.
Well, first, make the buildings one movie clip. Then copy and paste another set of buildings, (the same MC you just made) and move it right next/to the left of your first set of buildings. Third, highlight both of those mc's and make that yet another big MC. Then make it yet ANOTHER movie clip called Scrolling BG or something. Then go into the movie clip and turn on onion skin. Make sure one set of buildings is showing on the stage, then insert a key frame on frame 10. Insert a motion tween from frame one to ten, then drag your onion skin from frame ten alllllll the way back to frame 1. Now, on frame 10, just move your building set to the right so that the building set to the left ends up on the building set to the right from frame 1 using your onion skin. This should make it so that the buildings scroll to the right, if your guy is running to the left. Now to make the loop look better, insert a keyframe on frame 9 and drag it to frame 10. This will clear up some pausing in your loop.
Hope that helped, if not, I'll do it for you if you email me an .fla with your buildings in it.
Email btw... starogreATstarogreDOTcom, link is in my profile.