Is the extension compatible with iDEAL (Dutch payment processor)?

Yes. There is one additional step to perform when installing Fooman Surcharge, so that iDEAL can recognise that a surcharge has been applied: 

1. Edit the following file: app/code/core/Mage/Ideal/Model/Basic.php 
2. Add the following code at line 160: 

 if ($order->getBaseFoomanSurchargeAmount() > 0) {
 	$fields = array_merge($fields, array(
 		"itemNumber".$i => $order->getFoomanSurchargeDescription(),
 		"itemDescription".$i => '',
 		"itemQuantity".$i => 1,
 		"itemPrice".$i => -$order->getBaseFoomanSurchargeAmount()*100
 	));
 	$i++;
 }