I recently discovered that IE9 remembers the Compatibility View setting for a domain name, but does not remember the setting for an IP address, if the browser is closed and the option to “Delete browsing history on exit” is checked. The “Delete browsing history on exit” on is found under the General tab in Internet Options.

If you choose to view a website in Compatibility View, as a convenience to you, Internet Explorer will remember this choice and use Compatibility View the next time you visit the site. You can clear the list of websites you’ve chosen to display in Compatibility View by using the Delete Browsing History feature in Internet Explorer or the Compatibility View Settings dialog box.
http://windows.microsoft.com/en-US/internet-explorer/products/ie-9/windows-internet-explorer-9-privacy-statement

Update: Hmm, actually. I think I’m wrong about this, or there’s more to it than meets the eye.