Jan,
There is in fact a WPF bug, that we are unfortunately not getting much help from Microsoft to workaround, that causes bitmaps generated in memory with a longer aspect ratio (longer height, shorter width) to get this blurry effect. This in fact works much better in Silverlight. We are continuing to try to workaround this and hopefully soon have a fix.
But, there is also another scenarios where the print out could get blurry, which is when you try to print to a specific X by Y set of pages. We think that is not the case here, but just something fyi.
Thanks
RadiantQ Support