Starting from Context 2010.05.21 there is the libertine typescript already defined. It points to the fonts shipped with TeXlive2010 (they have a different name than the upstream). You have to define script=latn
if you want the ligatures in the Bold Italic font (until the upstream doesn't set a default script). See
http://archive.contextgarden.net/thread/20101108.150757.a6a9040d.en.html
% use microtypography \definefontfeature[default][default][protrusion=quality,expansion=quality,script=latn] \usetypescript[libertine] \setupalign[hz,hanging] \setupbodyfont[libertine,10pt] \starttext \input tufte \stoptext