I was thinking that things placed in the margin (\inmargin,\inright,\inleft) would not take up vertical space, but it seems to do just that inside an itemize.
Example:
\starttext
\textwidth=4cm
\hairline
%(1)
\startitemize[n,joinedup]
\item abc
\item cde
\stopitemize
\hairline
%(2)
% This seems to me the logical place for the remark.
\startitemize[n,joinedup] \inright{pqr} % < squeezing spaces does not help
\item abc
\item cde
\stopitemize
\hairline
%(3)
\startitemize[n,joinedup]
% But it has to be placed here to get the result wanted.
\item abc \inright{pqr}
\item cde
\stopitemize
\hairline
\stoptext
I wanted a remark in the right margin logically applying to the itemize as a whole.
Placing it as in (2) looked the right way, but then the next item is placed lower on the page. That becomes a problem when a nested itemize is put there resulting in an empty line at the first itemmarker. Placing the \inright on the first item of the inner itemize seems an illogical place to me. It is, of course, not really problem because trial (3) shows how to avoid this.
Still the question remains if there is some misbehaviour here in either the code for \inright or for \itemize? Therefore I am pointing this out just in case it might reveal a more or less subtle bug too more wiser ConTeXt-brains then mine.