There are some binary applications that require libpng.so.2 (e.g. http://www.ncbi.nlm.nih.gov/Structure/CN3D/cn3d.shtml). A libpng-1.0.18 ebuild should be added to portage that can live side by side with libpng-1.2.8 .
No ebuilds in portage need this one. If you need it for some apps that are not in portage then put an ebuild for the older version into your overlay.