I started to play with Syntax Highlighting today: I came across this : http://wordpress.org/extend/plugins/wp-syntax/ Seems to work nicely but you have to be in HTML Mode and if you switch back and forth you can loose your code snippet.
I ended up disabling the Visual Editor, which of course i can re enable for future posts. Below is a Test:
<div id="foo"> <?php function foo() { echo "Hello World!\\n"; } ?> </div>