[tex-live] Problem with TeX Live 2009's mpost + mfplain

[Scott Pakin]

I just upgraded to TeX Live 2009 and have already ran into a problem
with the new MetaPost.  I'm getting "Inconsistent equation" errors
when I try to use mpost with mfplain:

    $ mpost -ini '\input mfplain; dump'
    This is MetaPost, version 1.208 (kpathsea version 5.0.0) (INIMP)
    (/usr/local/stow/texlive-2009/texmf-dist/metapost/base/mfplain.mp
    Preloading the plain base, version 0.99: preliminaries,
     basic constants and mathematical macros,
     macros for converting units,
     macros and tables for various modes of operation,
     macros for drawing and filling,
     macros for proof labels and rules,
     macros for character and font administration,
    and a few last-minute items.)
    Beginning to dump on file mfplain.mem
     (mem=mfplain 2009.11.12)
    at most 803 strings of total length 4477
    3681 memory locations dumped; current usage is 1104&2542
    565 symbolic tokens
    Transcript written on mfplain.log.

    $ mpost -mem mfplain '\mode:=proof; prologues:=2; input cmr10'
    This is MetaPost, version 1.208 (kpathsea version 5.0.0)
    (/usr/local/stow/texlive-2009/texmf-dist/metapost/base/mfplain.mp
    Preloading the plain base, version 0.99: preliminaries,
     basic constants and mathematical macros,
     macros for converting units,
    ! Inconsistent equation (off by 2.77794).
    <to be read again>
                       ;
    l.230 mm*bpppix_=2.83464;
                                   pt*bpppix_=0.99626;
    ? x
    Transcript written on mfplain.log.

The same commands work fine using the TeX Live 2008 versions of everything:

    $ /usr/local/stow/texlive-2008/bin/i386-linux/mpost -ini '\input mfplain; dump'
    This is MetaPost, Version 1.005 (Web2C 7.5.7) (INIMP)
    (/usr/local/stow/texlive-2008/texmf-dist/metapost/base/mfplain.mp
    Preloading the plain base, version 0.99: preliminaries,
     basic constants and mathematical macros,
     macros for converting units,
     macros and tables for various modes of operation,
     macros for drawing and filling,
     macros for proof labels and rules,
     macros for character and font administration,
    and a few last-minute items.)
    Beginning to dump on file mfplain.mem
     (mem=mfplain 2009.11.12)
    at most 1619 strings of total length 27588
    3690 memory locations dumped; current usage is 1113&2542
    559 symbolic tokens
    Transcript written on mfplain.log.

    $ /usr/local/stow/texlive-2008/bin/i386-linux/mpost -mem mfplain '\mode:=proof; prologues:=2; input cmr10'
    This is MetaPost, Version 1.005 (Web2C 7.5.7)
    (/usr/local/stow/texlive-2008/texmf-dist/fonts/source/public/cm/cmr10.mf
    (/usr/local/stow/texlive-2008/texmf-dist/fonts/source/public/cm/cmbase.mf)
    (/usr/local/stow/texlive-2008/texmf-dist/fonts/source/public/cm/roman.mf
    (/usr/local/stow/texlive-2008/texmf-dist/fonts/source/public/cm/romanu.mf
    The letter A [65{/users/pakin/.texlive2008/texmf-var/fonts/map/pdftex/updmap/pdftex.map}]
    The letter B [66]
    The letter C [67]
                                  .
                                  .
                                  .
    128 output files written: cmr10.0 .. cmr10.127
    Transcript written on cmr10.log.

Any ideas what the problem is?  Is mpost 1.208 perhaps predefining
things that mpost 1.005 did not?

Thanks,
-- Scott