In both MKII and MKIV (using the latest standalone which I installed this morning), with
\setupheader[state=high]
\setuppagenumbering[alternative=doublesided]
\setuppagenumbering[location=,]
\setuphead[title][page={header,yes}]
\definehead[BookTitle][title]
\setuphead[BookTitle][before={\noindent\textrule},style=rm]
\setuphead[BookTitle][header=high]
\setupheadertexts[text]
[\cap{A.\ U.\ Thor}]
[\pagenumber]
[\pagenumber]
[\it{The Journal}]
\starttext
\dorecurse{6}{\input knuth}
\page[makeup]
\BookTitle{{\em An Indubitably Important Book} by A.\ U.\ Thor}
\dorecurse{10}{\input knuth}
\page[makeup]
\stoptext
the problem remains, that is, the header on the page preceding the page with the title is not suppressed. The same is true when I have \setuphead[title][page={blank,yes}], except then there are no headers at all after the page with the title on it. Does it work for you?