Just finished and eager to share - I added a new syntax definition to the Text_Highlighter PEAR package (see also here). It's for highlighting code written in the SAP's own ABAP programming language.
A live demo is available at the hiliteme.com site, just pick ABAP from the drop-down of programming languages. Any feedback is appreciated, because it's a brand new thing and may have bugs or incompletenesses (is that a word?). So feel free to highlight your ABAP code and post it to blogs or forums.
The implementation wasn't hard, the Text_Highlighter package is made to be extended and even provides the tools for that. All you need to do is create an XML file that contains keywords and other syntax rules, such as formats of the comments and so on. Then there is a command line tool that takes the XML file and generates a class out of it. The class is later on used when highlighting. Here's the XML file in case you want to improve on it and generate your own ABAP.php class: