Hi Hans,

the plain format from context calls a non-existing function
fonts.goodies.filenames.resolve(). It appears that the format was
not updated to include the expected code from fonts-gds.lua. [1]
Terminal output below.

Regard
Philipp

[1] The code was introduced to font-def.lua on November 16:
    http://repo.or.cz/w/context.git/commitdiff/4f08d2072378de7bac2c7992876b8f7012f7be78



$ luatex -fmt luatex-plain.fmt 
This is LuaTeX, Version beta-0.70.1-2011051923 (rev 4277) 
  \write18 enabled.
**\relax
(./luatex-basics.tex) (./luatex-fonts.tex <push namespace> <./luatex-fonts-merg
ed.lua>(using cache: /home/laokoon/context/tex/texmf-cache/luatex-cache/generic
)
! LuaTeX error ./luatex-fonts-merged.lua:12701: attempt to index local 'fontgoo
dies' (a nil value)
stack traceback:
    ./luatex-fonts-merged.lua:12701: in main chunk
    [C]: in function 'dofile'
    ./luatex-fonts.lua:107: in function 'loadmodule'
    ./luatex-fonts.lua:111: in main chunk
    [C]: in function 'dofile'
    <\directlua >:1: in main chunk.
l.136 }
        
? 
! Emergency stop.
l.136 }
        
!  ==> Fatal error occurred, no output PDF file produced!
Transcript written on luatex-basics.log.