It appears that there are different hyphenation patters when using oldstyle figures or not. Consider the following MWE:
\documentclass[a4paper,10pt]{scrartcl}
\areaset[current]{336pt}{600pt}
\overfullrule=5pt
\usepackage{mathpazo}
%\usepackage[osf]{mathpazo}
%\usepackage{fontspec}
%\setmainfont[
% Numbers={OldStyle,Proportional},
% ] {TeX Gyre Pagella}
\begin{document}
\noindent Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit, vestibulum
ut
\end{document}
Which generates an overfull hbox:

If we activate old style figures, that problem does not occur:

I have two questions:
What is going on here? I don't see any numbers in the text, so why would XeLaTeX generate different output?
Ultimately I want to use the Open Type Font TeX Gyre Pagella. If we generate this example the result is identical to the first picture, i.e. we get an overfull hbox. How can I achieve the same result as with osf-mathpazo?
mathpazoas well as with TeX Gyre Pagella. However the text font is not the same, so differences in character width and other aspects such as kerning can be expected. – egreg Mar 9 '12 at 9:55OSFusually stands for "oldstyle figures". Hence, please consider editing the title of your question to read "... oldstyle figures?" rather than "... old style fonts?. Old-style fonts tends to refer to fonts with "fancy" glyph shapes, such as a long-s -- a subject not intended to be brought up by your question, right? – Mico Mar 9 '12 at 11:43