Grey box code and lists

Using \colorbox from the color package instead would be better than a plain TeX macro:

\usepackage{color}
\definecolor{lightgray}{gray}{0.75}

\newcommand\greybox[1]{%
  \vskip\baselineskip%
  \par\noindent\colorbox{lightgray}{%
    \begin{minipage}{\textwidth}#1\end{minipage}%
  }%
  \vskip\baselineskip%
}

The result:

The solution doesn't allow any pagebreaks. You should use a package like framed or mdframed (new version is on the way ;-) I'm working on it. ):

\documentclass{article}
\usepackage[noframe]{showframe}
\usepackage{framed}
\usepackage{lipsum}
\renewenvironment{shaded}{%
  \def\FrameCommand{\fboxsep=\FrameSep \colorbox{shadecolor}}%
  \MakeFramed{\advance\hsize-\width \FrameRestore\FrameRestore}}%
 {\endMakeFramed}
\definecolor{shadecolor}{gray}{0.75}
\begin{document}
\lipsum[1]
\begin{shaded}
\lipsum
\end{shaded}
\lipsum[2]
\end{document}