Multicolumn within a newcommand error

\multicolumn must be the first thing in a table cell and a command with a star variant defined in the usual way hides \multicolumn after some unexpandable token. Without packages you can do in the following way:

\documentclass{article}
\newcommand{\topics}[1]{%
  \if*\detokenize{#1}%
    &%
  \else
    \multicolumn{2}{|p{2in}|}{#1}%
  \fi
}
\begin{document}
\begin{tabular}{|p{1in}p{1in}|}
  \topics{stuff} \\
  \topics* \\
  \topics{some more stuff} \\
  \topics* \\
  \topics*
\end{tabular}
\end{document}

This solution is built on the requirement suggested by Heiko in ! Misplaced \omit error.

etextools provides fully-expandable conditioning commands to allow \multicolumn to be considered as "the first element in the cell". This is not currently the case in your example, since \@ifstar is not fully expandable, while \FE@ifstar is a Fully Expandable version:

\documentclass{article}
\usepackage{etextools}% http://ctan.org/pkg/etextools
\makeatletter
\newcommand{\topics}[1]{%
  \FE@ifstar{#1}
    {test & test}% starred
    {\multicolumn{2}{|p{2in}|}{#1}\@gobble}}% non-starred
\makeatother
\begin{document}
\begin{tabular}{|p{1in}p{1in}|}
  \topics{stuff} \\
  \topics* \\
  \topics{some more stuff} \\
  \topics* \\
  \topics*
\end{tabular}
\end{document}