Gabapentin doesn't have FDA approval as a treatment for fibromyalgia pain, but it's being considered for that purpose. According to Science Daily:

Dr. Lesley Arnold, director of the Women's Health Research Program at the University of Cincinnati's College of Medicine, and her colleagues found those taking gabapentin at dosages of 1,200 to 2,400 mg daily for 12 weeks displayed significantly less pain than those taking placebo.

Patients taking gabapentin also reported significantly better sleep and less fatigue.

"While gabapentin does not have Food and Drug Administration approval for fibromyalgia, I believe this study offers additional insight to physicians considering the drug for their fibromyalgia patients," said Dr. Stephen Katz, NIAMS director.

The Science Blog reports:
Although gabapentin has little, if any, effect on acute pain, it has shown a robust effect on pain caused by a heightened response to stimuli related to inflammation or nerve injury in animal models of chronic pain syndromes. Researchers have suspected that it might have the same effect in people with fibromyalgia. The new research, published in the April 2007 edition of Arthritis & Rheumatism, indicates the suspicions were correct.