I have seen description-list-with-right-alignment-of-labels that explains, how to align description labels on the right. However the presented solution sets the lengths globally whereas I look for a local solution, meaning to specify the width for each environment separately. Is there a way telling each description environment what the broadest item is:
\documentclass{article}
\usepackage{enumitem}
\SetLabelAlign{parright}{\parbox[t]{\labelwidth}{\raggedleft#1}}
\setlist[description]{style=multiline,topsep=10pt,leftmargin=5cm,font=\textbf,align=parright}
\begin{document}
\begin{description}
\item[aaa] some text
\item[bbbbbb] some more text
\item[ccccccccc] and some text
\end{description}
\begin{description}
\item[aaaaaaa] some text
\item[bbbbbbbbbbbbbbb] some more text
\item[ccccccccccccccccccccccc] and some text
\end{description}
\end{document}

listenvironment to typeset thedescriptionlist. Let the environment take an option that takes the item with the maximum width. Use it to set the\labelwidthat the start of the environment (using the style for the labels). You also have to redefine\makeitem, which should use the label, and set it in the right style, right justified in an\hbox to \labelwidth(or equivalent). It's not too much work, but keep track of how thelistenvironment positions the labels. – Mar 02 '12 at 08:39enumitemallows you to set parameters both globally (as in your MWE) and locally (as in\begin{description}[style=...])? – DevSolar Mar 02 '12 at 08:40\makeitemshould be\makelabel. – Mar 02 '12 at 09:01