Here's the solution:
- Quit Preview.app if open.
- Open ~/Library » Preferences » com.apple.Preview.plist with a plain text editor.
- Search for thumbnail; it should find the PVGeneralThumbnailSize key.
- Replace the integer right underneath (which varies depending on what you set your preferred thumbnail size to) to 0.
- Save the change made and close.
[robg adds: The plist file is stored as a binary, so editing it in a plain text editor probably won't work. If you have the Developer Tools installed, you can just double-click it. You could also use PlistEdit Pro or Pref Setter to edit the file. If you're Terminally-inclined, you can use the process described in this hint to convert the file from binary to text for editing, and then back to binary again when you're done.]

