After seeing syntax highlighting by a friend, i decided to update my serendipity also. After a little bit of trying, i am now using "Prettify for S9Y (serendipityeventprettify)".
How to use it? Quite simple, install it and use "<pre class="prettyprint lang-foo">your code</pre>" to cover your codeblock. The following languages are supported right now:
lang-bsh lang-c lang-cc lang-cpp lang-cs lang-csh lang-cyc lang-cv lang-htm lang-html lang-java lang-js lang-m lang-msml lang-perl lang-pl lang-pm lang-py lang-rb lang-sh lang-xhtml lang-xml lang-xsl