(I realize the original post is from 2005, but I'd like to weigh in on this one, anyways.)
There is no doubt that getting into an Pain Fellowship from a psych background will be very difficult for even high quality applicants. However, given the fact that the psychological aspect of chronic pain is extremely prevalent, and largely ignored, I think we need more psychiatrists going into Pain, interventional included. If you think you have good aptitude for procedures, take a Pain elective in residency, get some basic procedures under your belt, maybe even take an ASIPP cadaver course, and you might have a shot if you apply widely enough. That being said, if you are sure you want to do pain, anesthesia is far and away your best bet statistically if you are at the medical student stage. In my opinion, the belief that you have to spend 4 years in general anesthesia to learn interventional Pain procedures is a myth. If you go to a quality fellowship like the one I graduated from (2 days ago), you'll do hundreds of epidurals, medial branch blocks, and other bread and butter procedures. If you're lucky, you may even do a few dozen implants.
How many 2 minute epidural steroid injections do you have to do to be competent? 300? No, but you'll do that many in a good (and highly interventional) fellowship. If an anesthesia intern can learn loss of resistance technique, so can you. For better or for worse, pain is multidisciplinary, and will become more so in the future.
{ Disclosure: I graduated an ACGME accredited fellowship, will take the ABA board exam this year and I come from the specialty category of "other", that is: non-anesthesia, non-PMR, non-neuro, non-psych, which some fellowship directors
claim they don't know is possible. Oh, I almost forgot, I had treated an estimated 21,000 patients with pain, prior to fellowship, but I know that doesn't mean anything
}