Here's the steps I took:
1) I started IE8b2 and checked the Compatability View Settings, several websites were listed and I closed the settings window
2) Opened regedit and looked at the HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\BrowserEmulation\ClearableListData\UserFilter entry, which existed and contained data
3) I ran CCleaner version 2.12.651 with no custom keys cleaned (while IE8 was still open)
4) Re-checked the Compatability View Settings, the list was now empty
5) Refreshed regedit and the key was gone