"Page is customized" in SPD 2007

Overview

I come across a situation that the "Edit Page" menu item is missing for certain page in a SharePoint Site. Later I found that it is caused by "Customized Page" created by SharePoint Designer (SPD).
If want to enable the "Edit Page" function of customized page, you can navigate to the page in SPD, right-click on it and select "Reset to Site Definition". However, this action will remove the customized page content.
Links below talk more about "customized page", "customized files" in WSS 3.0 & MOSS 2007

Reference

1. Understanding and Creating Customized and Uncustomized Files in Windows SharePoint Services 3.0
2. Reset a customized page to the site definition
3. 關於 SharePoint Designer 的利與弊