There are two ways until 2012 to become ACGME board certified in neuromuscular medicine:
Straight off the website from ABPMR:
Under temporary criteria, a practice track will be available for the first five years the examination is offered to ABPMR diplomates (2008-2012). Beginning with the 2013 examination, all applicants will be required to complete one full year of training in neuromuscular medicine in an ACGME-accredited program.
1. Satisfactory completion of 12 months in an ACGME-accredited fellowship. At least six months must be spent in clinical care of patients with neuromuscular disorders. The remaining six months of the fellowship will be flexible and may be spent studying related fields such as medical genetics, muscle pathology, electrodiagnostic medicine, or research. Physiatrists interested in this subspecialty may satisfy the requirements with six months' training in clinical neuromuscular medicine and six months in an EMG lab. The training program must occur after completing residency and must be completed by the August 31 that precedes the examination date.
OR
2. Under temporary criteria (available though the 2012 examination), ABPMR diplomates who consider themselves neuromuscular medicine specialists by virtue of time and effort may take the subspecialty examination by meeting the following criteria: Diplomates must have a minimum of 25% of professional time devoted to neuromuscular medicine for a minimum of two years, or one year of non–ACGME–approved fellowship training in Neuromuscular Medicine.
As of right now, there are very few if any ACGME PM&R neuromuscular medicine fellowships. I believe all of them are under Neurology. Maybe in the near future this may change. Same goes with the new Hospice/Palliative Medicine subspecialty, all ACGME fellowships are mostly under internal medicine.