For the following code:
\begin{tikzpicture}
\matrix [matrix of nodes, column 2/.style={red}]
{
8 & 1 & 6 \\
3 & 5 & 7 \\
4 & 9 & 2 \\
};
\end{tikzpicture}
This picture is created:

I also tried with:
\begin{tikzpicture}
\matrix [matrix of nodes, column 2/.style={rectangle,draw=black,fill=gray,minimum width=8em}]
{
8 & 1 & 6 \\
3 & 5 & 7 \\
4 & 9 & 2 \\
};
\end{tikzpicture}
The result is shown below:

It seems that some styles like red and minimum width are applied. Why are rectangle, fill and draw ignored?
How can I use them in matrix of nodes?

minimum widthis not applied andcircleis applied instead ofrectangle. I am therefore guessing thatrectangleis also applied. – ipavlic May 16 '11 at 14:32