Here's a possible solution; the \myboxed command surround its contents with a frame having the requested specifications; the syntax is:
\myboxed[<length>][<color1>][<color2>]{<contents>}
<length> controls the width of the frame (default=2pt); using the second and third optional arguments you can change the colors used (defaults=black and gray):
\documentclass{article}
\usepackage{xparse}
\usepackage{tikz}
\newlength\unit
\NewDocumentCommand\myboxed{O{2pt}O{black}O{gray}m}{%
\setlength\unit{#1}
\begin{tikzpicture}
\node[inner sep=0pt] (a) {#4};
\fill[#2] (a.south east) --
([xshift=\unit,yshift=-\unit]a.south east) |-
([xshift=-\unit,yshift=\unit]a.north west) --
(a.north west) -|
(a.south east) --
cycle;
\fill[#3] (a.south east) --
([xshift=\unit,yshift=-\unit]a.south east) -|
([xshift=-\unit,yshift=\unit]a.north west) --
(a.north west) |-
(a.south east) --
cycle;
\fill[#3] ([xshift=\unit,yshift=-\unit]a.south east) --
([xshift=2*\unit,yshift=-2*\unit]a.south east) |-
([xshift=-2*\unit,yshift=2*\unit]a.north west) --
([xshift=-\unit,yshift=\unit]a.north west) -|
([xshift=\unit,yshift=\unit]a.south east) --
cycle;
\fill[#2] ([xshift=\unit,yshift=-\unit]a.south east) --
([xshift=2*\unit,yshift=-2*\unit]a.south east) -|
([xshift=-2*\unit,yshift=2*\unit]a.north west) --
([xshift=-\unit,yshift=\unit]a.north west) |-
([xshift=-\unit,yshift=-\unit]a.south east) --
cycle;
\end{tikzpicture}
}
\begin{document}
\myboxed{\begin{tabular}{l|l}
A & B \\
\hline
C & D \\
\end{tabular}}\quad
\myboxed[4pt]{\begin{tabular}{l|l|l}
A & B & C \\
\hline
C & D & E \\
\hline
F & G & H \\
\end{tabular}}\quad
\myboxed[6pt][black!80][gray!50]{\begin{tabular}{l|l|l}
A & B & C \\
\hline
C & D & E \\
\hline
F & G & H \\
\end{tabular}}
\end{document}

\usetikzlibrary{fadings}and then\tikz\node[draw=black,ultra thick,double=gray,inner sep=.3pt+\pgflinewidth] [postaction={path fading=south,fill=white}] {\begin{tabular}{l|l} ...produces some effects- it's not quite what you've described though – cmhughes Nov 26 '12 at 22:43.pngformat and uploading it here; I hope it's OK. – Gonzalo Medina Nov 27 '12 at 15:34