tikz-pgfverbatim
Possible Duplicate:
Problem with TikZ, beamer and verbatim
Is it possible to use verbatim-like environments inside a tikz node? This MWE gives an error:
\documentclass{article}
\usepackage{tikz}
\begin{document}
\tikz{
\node{\begin{verbatim}a_b\end{verbatim}};
}
\end{document}
The problem with your code is that the \tikz command itself is not overlay-aware. (I think that it would be quite difficult to make it so.) So the \tikz stuff is getting executed on every slide, but the yyy node doesn't exist until the second slide so it doesn't know what to do with it. If you shift the <2> to the \draw instead then it works.
\documentclass[12pt]{beamer}
\usepackage{tikz}
\begin{document}
\begin{frame}
\begin{overprint}
\begin{tikzpicture}[remember picture]
\node<1->[draw] (xxx) at (0,0) {Spam};
\node<2->[draw] (yyy) at (1, 1) {Eggs};
\end{tikzpicture}
Some text, tra la la.
\tikz[remember picture] \draw<2>[overlay,->] (xxx.north) -- (yyy.south);
\end{overprint}
\end{frame}
\end{document}
You can use \Verb from the fancyvrb package:
\documentclass{beamer}
\usepackage{tikz}
\usepackage{fancyvrb}
\usetikzlibrary{arrows}
\begin{document}
\begin{frame}[fragile]{P}
\tikzstyle{cell} = [rectangle, draw, thin , fill=black!10 , minimum size=5mm]
\begin{tikzpicture}[node distance=3cm, auto,>=latex', thick, overlay]
\path[->,draw] node[cell] (p) {Text at \Verb!p!};
\end{tikzpicture}
\end{frame}
\end{document}
As Caramdir has correctly suspected in a comment, if another document class (article, for example) is used there's no need to use \Verb and the standard \verb command will work.
You should also consider using \texttt{p} instead of \verb!p! (as Torbjørn T suggested in his comment) if your intention is just to use a mono-spaced font for the "p" character.
Best Answer
I get
missing itemerror with your MWE. However, it seems to work withlistingspackage. If you would allow it, then try the code below.