Hi,

Really bad things happen if the variable name in metapost contains an 
underscore. It took me quite a while to isolate this problem.

Minimal example:

\startMPinclusions
   input rboxes ;
\stopMPinclusions

\starttext
\startMPpage
   rboxit.A (textext("A")) ;
   rboxit.B_1 (textext("B")) ;
   rboxit.C (textext("C")) ;

   A.c = origin ;

   xpart B_1.c = xpart C.c = xpart A.c + 3cm ;
   ypart B_1.s - ypart C.n = 1cm ;

   drawboxed(A,B_1,C) ;
\stopMPpage

\startMPpage
   rboxit.A (textext("A")) ;
   rboxit.B (textext("B")) ;
   rboxit.C (textext("C")) ;

   A.c = origin ;

   xpart B.c = xpart C.c = xpart A.c + 3cm ;
   ypart B.s - ypart C.n = 1cm ;

   drawboxed(A,B,C) ;
\stopMPpage
\stoptext


See attached output. The label of box B is placed at origin rather than at 
the correct location!

I am using  2010.08.17 and luatex 0.61 (too close to a deadline to 
update).

Aditya