Error reporting disappeared in PHP5 when using error_reporting "E_ALL" in VirtualHost

It has been my understanding, since a few years back, that setting
php_admin_value error_reporting "E_ALL"
inside my Apache's VirtualHost configuration actually permitted to set the error reporting to all types of error. Recently, upgrading Ubuntu from 8.04 to 8.10 (I don't know exactly which versions of PHP5 that actually meant), several of our team have seen their error reporting disappear... I first thought this was related to XDebug, installed on all our computers, but disabling it didn't fix anything. After 2 hours of searching for pretty much any topic related