I'm currently running Snow Leopard and Acrobat is currently set as my default pdf reader.
Its not just the pdf files that are showing up as an icon preview, its any file that's on the computer. The open/save dialog box clearly displays all icons in icon preview even though all options for icon preview has been turned off in the preferences.
Its not a big thing, I've learned to accept it, for now, its just the way it is, although it seems like a glitch to me.
It would certainly be easier when you need to attach a word or pdf document to view the default icon as they clearly stand out from the generic looking icon previews...
Do a "Save As" in Safari or any other application with icon preview turned OFF, save the file to your documents folder and you'll notice any files that are in the directory will appear in icon preview in the open/save dialog box when you're attempting to save the document.
The same goes when attempting to attach a file to an email, all the available files to choose from are displayed as an icon preview.
Thanks