Larry Rosen replied:
Because of the very confusion that is reflected in the discussion you referred to, my two new licenses (the The Open Source License and the The Academic Free License) make "perpetual" grants. I believe in being explicit.
Note that, for public policy reasons, under the Copyright Act all licenses are terminable "at any time during a period of five years beginning at the end of thirty-five years from the date of execution of the grant...." 17 U.S.C. 203(a)(3). This is true even if the license states that it is "perpetual." Even in that rare situation, though, derivative works prepared before termination may continue to be used but no additional derivative works may be created.