My sitemap is located at
http://www.spoenk.nl/beeldend/index.php?sitemap

It seems that Google is indexing the map, but it gives me one error message. I have no idea what is's about :

"This URL is niet allowed for a sitemap at this location" on line 3.

Me neither. You should first update (not sure if there was work on this in 1.4.0.3) but for 1.4.1 the sitemap has been reworked a little. You should try that as well.