test.php:
<?
    header('Content-Type: text/plain');

    echo "\narg_separator.input: ";
    print_r(ini_get('arg_separator.input'));

    echo "\n\$_GET: ";
    print_r($_GET);

call this either with 
test.php?foo=1&bar=2
or
test.php?foo=1;bar=2

output:

arg_separator.input: ;&
$_GET: Array
(
    [foo] => 1
)

apache-1.3.2[26]