I was about to write my own, when I found an older article from phoboslab. Thanks Dominic for saving me some time ;) Its not perfect, but close enough.
A simple Syntax Highlighting Class that does just that. The class was not working with PHP 5.4.x+, as it uses preg_replace() with the /e modifier.
It will not cover all, but its better than nothing :) I will also add a section to my my AMP tweaks article to showcase the integration of Geshi.
Here an updated version using the preg_replace_callback() function.