As of WordPress 3.1, some post and page editing options have been hidden, including the Except field.
While WordPress moves closer and closer to becoming a full featured CMS, it still requires many work-arounds and around-about ways of working to fit it’s many square edges into the more typical CMS round hole. The Excerpt field is one of these—nominally a field to add an excerpt about a post which can be used as a teaser when theming, it can also be used as a secondary content field of the type that Drupal’s Field Module cooks for breakfast, lunch and dinner.
As of WordPress 3.1, rightly or wrongly the Excerpt field—together with Custom Fields and several other essential fields—has been hidden from view by default in the Post & Page edit Administration Panels, and to be revealed must be explicitly turned on in the Screen Options tab.
According to the WordPress Codex itself:
Note: As of WordPress version 3.1, some screen options on the Post & Page edit Administration Panels are hidden by default. The Excerpt field is hidden by default if it has not been used before.
A simple fix to a modification one presumes intended to simplify the user experience, but a modification nonetheless sweeping enough to cause the unwitting front-end developer sleepless nights when unnoticed.