Since I was unable to get the spam protection working in all browsers I had the normal custom comments form on my blog. The last few weeks I've been getting more and more spam posts. So I've decided to remove the custom comments form and to replace it with the Blogger popup version.
An other down side to the custom form was that you wouldn't get feedback when you used HTML tags that aren't allowed by Blogger. So no hard feelings. It's a change for the better from a usability point of view as well.