Having finally got libboost-mpi-dev and the version specific packages into Debian and thence Ubuntu, it seems the Ubuntu management have decided it's far too much trouble and have now removed all Boost.MPI packages from Ubuntu, cf. the notification and proof it's not in Maverick. Debian are going the other way and providing backports of Boost.MPI so all versions of Debian support it, cf. proof it's in all version in one form or another.
The conclusion is clearly that whilst Debian are supporting all tools associated with multicore systems and HPC programming, the Ubuntu management have decided that C++ and MPI are an irrelevance to the future of Ubuntu.
Conclusion will be drawn. Mostly with the blue pencil of censorship.
If you care about having Boost.MPI in Ubuntu please visit this bug report and add your comment/support.