You need to set the page style after \maketitle since \maketitle sets the page using \thispagestyle{plain} (which overwrites your request for it to be empty):
\documentclass[a4paper,10pt,english]{article}
\title{My title}
\begin{document}
\maketitle
\thispagestyle{empty}% Reset page style to 'empty'
Hello World
\end{document}
If you wish to suppress the page numbering for all pages within your article, you should add \pagestyle{empty} to your document preamble.
The article document class also supports the titlepage option, which typesets an entire page (a titlepage) when using \maketitle. Such a page is necessarily issued using \thispagestyle{empty}. That is, without any header/footer.
\pagestyle{empty}. – Tobi Jan 09 '12 at 20:07