A *lot* of people have a hard time getting source code to display correctly. It seems as though input is taken verbatim, then spat out at the browser, which means that if you want a less than or greater than sign to show up right in your message, you'll have to use the HTML things for them: > for >, etc.
It seems like it would be better to take input verbatim, then escape the HTML stuff when sending to the browser. Then you can simply type "#include <stdio.h>" and when sending the message to a browser, change the '>' and '<' characters to the HTML entities > and <. The problem with this, of course, is that you could no longer include HTML formatting in your messages, which a lot of people take advantage of.
An alternative idea: a "codebox" of some sort implemented with a form text area. A special tag in the message could set it off, then everything inside that tag would be "escaped" as described above, and displayed as the default text in a text area similar to the Message textbox used when posting a message.