Is it possible to override the default session notices?
If you place a PHP file in craft/translations named after a locale ID and returning an array, those values will be used instead.

craft/translations/en.php

&lt;?php

return array(
    'Logged in.' =&gt; 'Good to see you.',
);


Craft's own CP translation files can be found in craft/app/translations.

You can read more about it here: http://buildwithcraft.com/help/static-translations