Syntax highlighting was added for the WordPress theme and plugin editors in 2.7, but was removed in 2.8.1 because of compatibility issues. Enable CodePress adds the functionality back.
Enable CodePress is broken as of WordPress 3.0 but I hope to eventually release an update that will introduce a new syntax highlighter.
To install, simply download the plugin and upload it to your plugin directory. Activate the it in your WordPress administration console and you’re good to go. No configuration needed.
Note: Enable CodePress doesn’t fix the compatibility issues that caused syntax highlighting to be disabled. If you have problems with the code editor, just disable the plugin to get the regular editor back.