To get All-in-One Event Calendar to follow standard WordPress directory structure, having theme folders in the themes directory, then a couple of simple edits to the file all-in-one-event-calendar.php will do the trick. Note: You need to edit the file before you activate the plugin, or if you do it after you have activated, then you will need to manually delete the existing theme folder from the wp-content main directory.
My real purpose in doing this was a nagging stubbornness to demonstrate that putting the theme files in the themes directory is a viable option. For sane users I don't think it makes a wit of difference and you would have to re-edit the all-in-one-event-calendar.php file again after every upgrade. Not being quite sane, I intend to do just that.
Anyway, I've been running with these edits for about a week on the newest version, 1.7, and have seen no ill effects. In fact 1.7 is definitely an advance on 1.5 well worth having.
define( 'AI1EC_THEMES_ROOT', WP_CONTENT_DIR . '/' . AI1EC_THEMES_FOLDER );
define( 'AI1EC_THEMES_ROOT', WP_CONTENT_DIR . '/themes/' . AI1EC_THEMES_FOLDER );
define( 'AI1EC_THEMES_URL', WP_CONTENT_URL . '/' . AI1EC_THEMES_FOLDER );
define( 'AI1EC_THEMES_URL', WP_CONTENT_URL . '/themes/' . AI1EC_THEMES_FOLDER );