is it upper back pain or lower back pain?
Have you had your bike professionally fitted? I was getting some nasty pain between my shoulder blades and went to my LBS to have the bike fit checked; it turned out that the handlebars were too low, and also that the grips were too far apart for me. The guy at the LBS moved my controls in a bit and cut the bars to fit me, and I haven't had a problem since. It was also very inexpensive - only about $10 for him to go over the whole bike and set it up right for me.
Hope that helps some!