The WP-Syntax plugin allows easy insertion of code in blog posts, using GeSHi (Generic Syntax Highlighter). It supposedly supports over one hundred programming languages, including including PHP, HTML, CSS, Java, C, Lisp, XML, Perl, Python, and assembly.
<pre lang="html" line="1" escaped="true"> </pre>
The line=”1″ directive tells WP-Syntax to insert line numbers. For this example, to nest xml inside xml, set escaped=”true”, and use html entities.