I have experienced segfaults and bus errors with the following configuration: FreeBSD4.4, Apache 1.3.26, PHP 4.2.2, GD-1.8.4, PDFlib 4.0.1. The apache process crashed when calling the imagepng function, but it didn't crash when calling the imagejpg function, or imagecreatefrompng...
Some wasted hours (lots) later, in which I have tried to recompile gd, libpng, php, libjpeg, what-not, I have found the following advices:
http://bugs.php.net/bug.php?id=16841
So the problem was not with the png library, but rather with the PDFlib. Even though all the threads led to a png-problem... so I have simply upgraded to PDFlib 4.0.3 (w/o any special configure arguments; --with-libpng didn't work anyways), recompiled PHP, and now everything works (imagepng, pdf creation, etc.).
Hope this helps,
bogdan