Homebrew can't adapt to other versions of gcc using command line options. You can easily override the older compiler, though, if you edit the open-mpi and boost formula. For example, you can add a few commands after the "def install" in open-mpi.rb:
def install
# Force compilation with gcc-4.6
ENV['CC'] = '/usr/local/bin/gcc-4.6'
ENV['LD'] = '/usr/local/bin/gcc-4.6'
ENV['CXX'] = '/usr/local/bin/g++-4.6'
# Compiler complains about link compatibility with FORTRAN otherwise
ENV.delete('CFLAGS')
ENV.delete('CXXFLAGS')
That worked for me on Lion. Good luck.
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