NBS stands for "Not Build from Sources". They are binary packages that are no more built from a source package anymore (for example, because they are superseded from a newer version), but are kept in the archive, since a number of packages still depends on them.
An appreciated activity involving NBS is to rebuild packages that depends on them, so that the NBS has no more reverse dependencies on it and can be removed from the archives. Frequently, a rebuild will cause a selection of new binary dependencies, and the package will work, although sometimes adjusting debian/control is needed, and more rarely, one needs to port the package to work with the new system.
An updated list of NBS is available at [http://people.ubuntu.com/~ubuntu-archive/NBS/]. For each package that NBS, the list of packages that depends on the NBS and needs to be ported is provided. Once everything is confirmed as ported (and the list reaches 0 packages), the NBS can be removed from the archive, making everything cleaner.
libmpich1.0c2 is a NBS superseded by libmpich1.0ldbl. These packages still depends on libmpich1.0c2:
-- hardy/universe amd64 deps on libmpich1.0c2: illuminator-demo libluminate7 libpetsc2.3.2 -- hardy/universe i386 deps on libmpich1.0c2: illuminator-demo libluminate7 libpetsc2.3.2 -- hardy/universe sparc deps on libmpich1.0c2: illuminator-demo libluminate7 libpetsc2.3.2
In this case, we have three binary packages to inspect: illuminator-demo, libluminate7, and libpetsc2.3.2. Looking at a hardy cache, we can discover that those come from two sources: illuminator and petsc.
apt-cache showsrc petsc | grep Depends shows:
$ apt-cache showsrc petsc | grep Binary Binary: libpetsc2.3.3-dbg, libpetsc2.3.3-dev, petsc-dev, petsc2.3.3-doc, libpetsc2.3.3
Our reverse dependency for petsc is libpetsc2.3.2, which means we don't really care, as the new source doesn't use it, but we need to NBS libpetsc2.3.2 before we can NBS libmpich1.0c2
Next step is to inspect the binary packages to see if they have alternatives defined in the dependencies. In this case, there are hard-dependencies for libmpich1.0c2 for both illuminator-demo and libluminate7.
Next thing to try is a rebuild. Update the changelog with a <version>-<debian>build1 entry, and report the transition you are processing in a form like:
* rebuild for libmpich1.0c2 -> libmpich1.0ldbl and libpetsc2.3.2 -> libpetsc2.3.3
Don't touch any other files in the package, generate a new source candidate, and try a test build.
If the package rebuilds fine, the transition is complete. Sometimes, anyway, the transition brings to a FTFBS that must be tackled and fixed before the NBS can be removed from the archive.
Once reverse dependencies from the NBS list are zeroed, announcing that on #ubuntu-devel would be greatly appreciated, so that an archive-admin can remove the NBS.