Yup, I got a similar letter (from Gibson & Sharps, supposedly, and Trover Solutions), actually naming my health care provider, and specifying a treatment date and a doctor's name (which I haven't looked up to see if it's listed on my provider's website).  The date is a day when one of us had a different kind of appointment & doctor, in actuality, and somehow they had my name and my wife's name, date of appt, health provider's name. So somehow some medical information has been shared; just enough to make someone (a victim) think it's official.  Or maybe they scan those reminder postcards and/or emails that get sent, somehow having access to them?
  The letter says "Trover Solutions has sent you three prior inquiries about the treatment referenced above, and you have failed to respond to any of them."   etc., etc.   Next thing you know, they'll be asking for one of those green dot cards for cash.        I'm not responding, but will notify my health care provider. There's also a footnote: "Trover Solutions is a business associate of [my health care provider's name]" etc., etc.   Makes it sound real, doesn't it?!
  A big thanks to those of you who have posted info on your experiences. I had a feeling that if I entered the phone number (listed in the letter) into 800notes I'd see something about a scam; and sure enough!

Yes, I got the same letter. I called just to get them off my back. I think they are ambulance chasers. They are looking to get money for a claim in a "third party" doctor visit. Mine was not one. They dismissed me very quickly and said that me calling took care of the matter.

Who owns the number?

Got a letter dated Dec. 12, 2011 from Gibson & Sharps, Attorneys at Law:  "This firm represents Health Care Recoveries, subrogation vendors for Kaiser California North.  You may wish to review your health plan contract to determine any legal obligation you might have to provide certain information in connection with the claim referenced above.  Please contact Health Care Recoveries' Customer Service Department immediately at 1-800-288-1576 to resolve this matter and avoid further inquiries.  For your convenience the various hours of operation are listed below...Sincerely, S. S. Quaye".  Across the bottom of the page, "Inquiries made by Healthcare Recoveries on behalf of your health plan are in full compliance with HIPPA"
Noted:  The letter was sent under bulk rate postage and the page has all kinds of the typical undecypherable number codes printed at top of my address line and across bottom of page.  This is similar to all those innocuous credit card offerings we all receive.
Note the vailed ambiguity in the above text with the use of words, "may", "any", "might" and "certain".

THIS IS PHISHING PLAIN AND SIMPLE.  IF THERE ARE ISSUES WITH YOUR HEALTH CARE PROVIDERS SIMPLY CONTACT THEM DIRECTLY.  IF THOSE ISSUES ARE NOT RESOLVED GET YOUR OWN TRUSTED PRIVATE COUNCIL.  KEEP YOUR PRIVATE, PERSONAL INFORMATION TO YOURSELF.  DO NOT SHARE YOUR INFORMATION WITH THE ABOVE DISCUSSED PHISHERS.