Rui, does this exist also in PHP 5.x branches/HEAD?
If so and this is real bug, update the version. :)

Could you show me mbstring part of php.ini ?
And please show me the simple script to verify your ploblem.
I executed this tiny script, and if forks fine. 
(with Fedora Linux 5, PHP 5.1.5)
<?php
$string = "test?";
echo mb_detect_encoding($string,array('ISO-8859-1','UTF-8'));
// returns ISO-8859-1
?>

I read that 

mb_detect_encoding($string,array('ISO-8859-1','UTF-8'));

always return ISO-8859-1.

Try this;
<?php
$string1 = "test?";
echo mb_detect_encoding($string1,'UTF-8, ISO-8859-1');
// returns UTF-8
echo " ";
$string2 = "test?e";
echo mb_detect_encoding($string2,'UTF-8, ISO-8859-1');
// returns ISO-8859-1
?>
(php 4.4.0)

It is also happend in PHP 5.2.

Thank you for your bug report. This issue has already been fixed
in the latest released version of PHP, which you can download at 
http://www.php.net/downloads.php

If strict mode detection is used in PHP 5.2.3,
both results are ISO-8859-1.

<?php
$string1 = "test?";
echo mb_detect_encoding($string1,'UTF-8, ISO-8859-1', true);
// returns UTF-8
echo " ";
$string2 = "test?e";
echo mb_detect_encoding($string2,'UTF-8, ISO-8859-1', true);
// returns ISO-8859-1
?>