+1  A: 

This effect seems to be especially noticeable on the "Adobe" letters on the icon. Since I assume that the icon is some kind of graphic, this is not related to ClearType which is only marginally useful under WPF anyways, but a more general problem. One usual suspect is be pixel snapping.

David Schmitt
I was actually thinking more about the text in the ComboBox - it seems to be less clear and the edges seem to be less sharp than the text in the ListBox. Any ideas on that?
robintw
Regarding Text in WPF, I can only point you to the post I've already linked to: http://stackoverflow.com/questions/190344/wpf-blurry-fonts-problem-solutions/190521#190521 summary: current WPF text rendering quality is optimized for smooth animations, not pixel grid snapping.
David Schmitt
+1  A: 

It's because the images are not aligned with the pixel grid, you can either give everything explicit sizes and positions that are aligned or use an alternative image control, see this post on my blog:

http://www.nbdtech.com/blog/archive/2008/11/20/blurred-images-in-wpf.aspx

Nir
+1  A: 

The text blurriness on ComboBox when compared to ListBox is because ClearType is disabled on popups (including the one for ComboBox). This has been confirmed by Microsoft.

From the linked article:

In a popup, which is a separate Win32 window, there's no way of the WPF ClearType system knowing what pixels are behind (without doing various dubious and slow win32 tricks) so it's simply disabled.

Robert Macnee