no syntax highlighting when editing a node, only when viewing it?

Indeed, the GeSHi filter syntax highlighting is only applied when viewing a fully rendered node (which happens server side).

GeSHi filter does not provide live syntax highlighting in the node editing text field (which is client side).

If you want client side/live syntax highlighting, check out the javascript based syntax highlighting options: http://drupal.org/project/syntaxhighlighter, http://drupal.org/project/chili_highlighter, http://drupal.org/project/ajax_markup