Does the margin thing, but incapacitates packed and joinedup, which are ignored if the margin statement is there. So, I get indented items, but at the price of losing joinedup and packed.
\setupindenting[yes,medium]
\starttext
Foo bar bar foo. Foo bar bar foo. Foo bar bar foo.
\startitemize[joinedup,packed]
\item Foo bar
\item Foo bar
\stopitemize
Foo bar bar foo. Foo bar bar foo. Foo bar bar foo.
Foo bar bar foo. Foo bar bar foo. Foo bar bar foo.
\startitemize[joinedup,packed,margin=standard]
\item Foo bar
\item Foo bar
\stopitemize
Foo bar bar foo. Foo bar bar foo. Foo bar bar foo.
\stoptext