If unrecognized pragmas are ignored, then I don't see why boost developers should jump through a hoop to satisfy the "el-cheapo" compilers. In other words, if a compiler gives a warning about the #pragma-once, then let the programmer turn off the warning locally for the build, which is usually done in a single location within a makefile.

Well, what about the compilers that recognize #pragma once but don't implement it correctly? Or implement it as well as they can on their platform but somehow differ from other compilers? Also, there was once a version of GCC that would launch nethack or rogue when it encountered #pragma once. That's a perfectly legal thing for it to do (if not all that helpful). My point is that since we can't depend on the semantics of #pragma once, we ought not to use it. --Jeffrey Bosboom