According to otfinfo, greek small capitals are supported in Minion Pro.
otfinfo --script=grek -f MinionPro-Regular.otf
...
smcp Small Capitals
...
yet the following MWE
\documentclass{article}
\usepackage{fontspec}
\setmainfont{Minion Pro}
\begin{document}
\textsc{Small Capitals}
\textsc{Πεζοκεφαλαία}
\end{document}
results in:

What option should I pass to fontspec in order to have small capitals with Minion Pro for greek Glyphs?
.
otfinfothere must be small caps. Also on libre office there are small caps, but I don't know if they are real or not. – pmav99 May 12 '11 at 18:14\addfontfeature{Script=Greek, Letters=SmallCaps}too and the result is the same – henrique Jul 1 '11 at 0:01