Is there an FAA ruling about the airworthiness of piston engines with less than 60 on the compression test?
Long story short is that the manufacturers instructions take precedence over all. FAR Part 43 Appendix D requires that at each annual and 100 hr the compression be checked and that if there is "weak" compression further inspection of condition and tolerances of the engine have to be made. Now the term "weak" is kind of broad. FAA Advisory Circular AC43.13-1B states:


  If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine, and procedures in paragraphs 8-14b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected.


However, it also states:


  This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions. 


So the gist of it seems to be that if the compression is below 60/80 on a hot engine IN GENERAL, then yes it needs to be removed and inspected, UNLESS the manufacturers instructions state otherwise. Keep in mind Advisory Circulars are guidelines and not regulatory in nature although they are usually used as best practice guides. There is a great AVweb article The Savvy Aviator #13: Putting Compression In Context that has more in depth analysis.