For me, “Featured Image” was still missing from the Screen Options tab, even after clearing the cache and restarting the browser. In my case the problem was related to having a WordPress 3.2 Multisite install. With this setup, the Media upload buttons must be checked in Network Admin to enable the featured image meta box in the editor (these are unchecked by default). From the WordPress Codex:

To show the “Featured Image” meta box in mulsite installation, make sure you update the allowed upload file types, in Network Admin, Network Admin Settings SubPanel, Upload Settings, Media upload buttons options. Default is off.

Also: Have you looked in the Screen Options tab (top right on the Add Post / Edit Post page) to make sure ‘Featured Image’ is checked? It may not be set to show up by default but it should be there. Once you’ve checked the box it will remember your preferences.