Based on Charles Stewart's comment and Matten's answers, I make the following comparion. I suggest to follow the Stewart's suggestion. If we insist on using itemize environment instead of description, the bullet item will expands to the left crossing the left margin.

\documentclass[dvipsnames]{article}
\usepackage{xcolor}
\usepackage[showframe=true,margin=30mm]{geometry}
\usepackage{showexpl}
\usepackage{etoolbox}
\makeatletter
\patchcmd{\SX@codeInput}{xleftmargin=0pt,xrightmargin=0pt}{}
{\typeout{***Successfully patched \protect\SX@codeInput***}}
{\typeout{***ERROR! Failed to patch \protect\SX@codeInput***}}
\makeatother
\lstset{%
literate={ï}{}0
{»}{}0
{¿}{}0,
breaklines=true,
breakindent=0pt,
basicstyle=\ttfamily\scriptsize,
keywordstyle=\color{blue}\sffamily\bfseries,
commentstyle=\color{Green}\itshape,
stringstyle=\rmfamily,
showstringspaces=false,
backgroundcolor=\color{Yellow!30},
frame=single,
framerule=0.4pt,
rulecolor=\color{red},
framesep=3pt,
xleftmargin=3.4pt,
xrightmargin=3.4pt,
tabsize=2,%
explpreset={pos=b}%
}
\begin{document}
\begin{LTXexample}
\begin{description}
\item{[GMR85]} I am happy.
\item{[GMR86]} Are you happy?
\end{description}
\end{LTXexample}
\begin{LTXexample}
\begin{itemize}
\item[{[GMR85]}] I am happy.
\item[{[GMR86]}] Are you happy?
\end{itemize}
\end{LTXexample}
\end{document}
{description}rather than{itemize}with lexicon-like lists. – Charles Stewart Dec 07 '10 at 13:23bibliographyenvironment? – Caramdir Dec 07 '10 at 17:51bibliographyenvironment, but let's just say that there are technical reasons I don't want to do that now. (It's a long story!). As a short answer, you can just assume that I want an enumerate listing whose labels include brackets. (while they aren't citations.) – Sadeq Dousti Dec 07 '10 at 18:26{description}environment does not work in thebeamer. (Screenshot: http://s1.bild.me/bilder/311010/1151044untitled.PNG) – Sadeq Dousti Dec 07 '10 at 18:37