\def\ChapterCommand#1#2#3{#1 #2 #3}%
\setuplist[chapter]
[alternative=command,
command=\ChapterCommand]
\starttext
\placecontent
\startpart[title=aaa]
\startchapter[title=bbb]
\startsection[title=ccc]
\input knuth
\stopsection
\stopchapter
\stoppart
\stoptext
! Argument of \ChapterCommand has an extra }.
Perhaps
*** strc-lst.mkvi.new 2012-01-26 16:47:46.000000000 +0100
--- strc-lst.mkvi 2012-01-23 19:44:00.000000000 +0100
***************
*** 755,757 ****
\startsetups[\??listrenderings:\v!command]
! \def\p_command{\listalternativeparameter\c!command}%
\ifx\p_command\empty
--- 755,757 ----
\startsetups[\??listrenderings:\v!command]
! \edef\p_command{\listalternativeparameter\c!command}%
\ifx\p_command\empty
--
luigi