The current version of MPICH is 1.2.5.2 and was released on September 11, 2003. Reproducible: Always Steps to Reproduce:
Created attachment 25331 [details] mpich-1.2.5.2.ebuild mpich-1.2.5.2.ebuild
1.2.5.2 is already in portage (but masked).
comment 2 is right, this is already in portage. adust ACCEPT_KEYWORDS if you'd like to use it.
or (instead of adjusting ACCEPT_KEYWORDS) emerge with full qualified path: emerge /usr/portage/sys-cluster/mpich/mpich-1.2.5.2.ebuild