Given that my wife is a board-certified pain management Anesthesiologist and the fact that I have participated in the training of over 100 Anesthesiologists in my career, I can say that on the average, there is a lack of straight musculoskeletal training. It is changing at many programs. At UC Davis, they have hired one PM&R faculty member and have brought another (me) to specifically teach musculoskeletal experiences.
The amount of exposure clearly will vary from program to program from one time of year to another based upon patient load, insurance demands of the attending and the exposure of the attending to musculoskeletal examinations.
You REALLY need to decide if you're going to do 'Pain' or 'Sports/ Spine' early on, as the training backgrounds are very different, and could change your choice of residency.
Pain: Anesthesiology teaches you boatloads about the pharmacokinetics of meds, physiology of pain, complication management, procedures, etc. May have less musculoskeletal exposure.
PM&R usually requires additional training to learn the procedures, as there are not enough done in training to meet hospital requirements.
Sports/ Spine: Not usually done by Anesthesiologists - not out of the questions, but given the reduced musculoskeletal exposure, it is not usually done. PM&R is excellently suited in residency to prepare you for a fellowhsip here, as your first three years are spent dealing with the mechanics of injury (minor to catastrophic) and trying to recover to baseline as much as possible.
Hope these help...
Brian