Hello:
I am at amd64 lenny for scientific computation (i.e. no x-system) and
i386 squeeze for the desktop with graphics.
For lenny, yesterday I needed MPICH2 instead of openmpi already
installed. No deb package for lenny, only for squeeze. I compiled
MPICH2 for lenny.
However, the parallelized package that requires support from MPICH2
requests python2.6, which is unavailable in lenny. Installing that
from squeeze, even with apt-pinning is too risky for me, and I find
too difficult to compile python. There is no technical help in my
institution.
One can suggest: move to squeeze for scientific computation. Not a
solution: the packages for chemical computation are not updated
frequently, particularly non-commercial ones. Key packages for me do
not run on squeeze, only on stable.