list($year, $month) = explode('-', $start);
$sdate = strtotime("$year-$month-01");
list($year, $month) = explode('-', $end);
$edate = strtotime("$year-$month-01");

$dates = array();
for($i = $sdate; $i <= $edate; $i = strtotime('+1 month', $i))
{
  $dates[] = date('Y-m-d', $i);
}

Si le date start est au 2012-09-15 et le date end est au 2012-12-14 alors quand tu testeras le dernier mois tu auras 2012-12-15 <= 2012-12-14 == false.
A la limite si le jour n'est pas important met le au premier par exemple.

for($i = $sdate; $i <= $edate; $i += strtotime ('+1 month',0))

for($i = $sdate; $i < $edate; $i <= strtotime ('+1 month',0) )
								{
									$dates [] = date('Y-m-d', $i);//ligne 61
								}

$dStart=$_POST['date_debut'];
							$dEnd=$_POST['date_fin'];
							$nom=$_POST['nom'];
							function getDatesBetween($start, $end)
							{
								if($start > $end)
								{
									return false;
								}    
								
								$sdate    = strtotime($start);
								$edate    = strtotime($end);
								
								$dates = array();
								
								for($i = $sdate; $i < $edate; $i += strtotime('+1 month', 0))
								{
									$dates [] = date('Y-m-d', $i);
								}
								
								return $dates;
							}


							?>

<?php 
								$aDates = getDatesBetween ($dStart,$dEnd);
								foreach($aDates as $date)
								{ ?>
								<input type="hidden" id="nom" name="nom" value="<?php echo $nom; ?>" size="" />
								<input type="text" id="date" name="date[]" value="<?php echo $date; ?>" size="8" readonly="readonly" style="border: none;"  />
								<input type="text" name="nb[]" size="5" /><br>
								<?php } ?>