See [http://wiki.contextgarden.net/System_Macros/Branches_and_Decisions]
Block of code depending on a Lua value - example (by WS, 3/2011):
<texcode>
\starttext
\ctxlua{test = true}
Test is \ctxlua{commands.testcase(test)}{True}{False}.
\ctxlua{test = false}
Test is \ctxlua{commands.testcase(test)}{True}{False}.
\stoptext
</texcode>