Is it possible to override the default session notices? I couldn't find anything on here about this. Maybe I'm the only one, but for example when you log in there is a session notice with the message "Logged in.", but I want it to display something like "Good to see you" or something.

I can see the notices are set via Craft::t(), so I would assume they are translatable somehow, so can I do this through configuration files or a plugin init method or something?
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