Centering part entries in toc using memoir

You can redefine the \@part command as defined in memoir.cls; this command in in charge of producing the part entries in the ToC; the redefinition basically consists in introducing two \hfils to center the entries:

\documentclass{memoir}

\cftpagenumbersoff{part}
\makeatletter
\long\def\@part[#1]#2{%
  \M@gettitle{#1}%
  \phantomsection
  \ifnum \c@secnumdepth >-2\relax
    \refstepcounter{part}%
    \addcontentsline{toc}{part}{\hfil\thepart~#1\hfil}%
    \mempartinfo{\thepart}{#1}{#2}%
  \else
    \addcontentsline{toc}{part}{\hfil#1\hfil}%
    \mempartinfo{}{#1}{#2}%
  \fi
  \partmark{#1}%
  {\centering
   \interlinepenalty \@M
   \parskip\z@
   \normalfont
   \ifnum \c@secnumdepth >-2\relax
     \printpartname \partnamenum \printpartnum
     \midpartskip
   \fi
   \printparttitle{#2}\par}%
  \@endpart}
\makeatother

\begin{document}

\tableofcontents*
\part{Part A}
\chapter{Chapter a}
\part{Part B}
\chapter{Chapter b}

\end{document}

The above code will center the entries with respect to the text width minus the length \@tocrmarg (default value of 2.55em) reserved to typeset the page numbers for other sectional units using some extra space; you can change this, and center the entries with respect to the full text width by adding \hspace*{\@tocrmarg} just before the first \hfils in my example code above.

Hmm, that can be done with much shorter code:

\makeatletter
\renewcommand\partnumberline[1]{\hfil\hspace\@tocrmarg #1~}
\makeatother

(not entirely sure why though, it's late ...)