Maybe this has already been reported, and if it has, I'm sorry.
If you are on PHP 5.4, then your site will break due to this error:
PHP Fatal error: Call-time pass-by-reference has been removed in .../wp-content/plugins/mailpress/mp-includes/class/MP_newsletter_post_type_.class.php on line 10
That line is this:
if (method_exists(&$this,'init')) add_action('init', array(&$this, 'init'), 1);
You will have to edit this file and change
&$this on this line to
&). It should work after that. At least I haven't found anything else so far... If I do I will share it here.