Edit button sometimes disabled on custom layout pages
Recently, I was working with a client to convert their public web site from CMS to a MOSS Publishing Portal. All of the layout pages are customized to create a very non-SharePoint look and feel. I noticed that on some of the pages the "Edit" button was available and on others it was disabled. After checking with the usual suspects (page checked out by another user, missing content placeholders, etc...) and a little side-by-side comparison, I found out that the pages with a disabled "Edit" button did not include any publishing content placeholders on the page when in the normal display mode. These pages only included "hidden" page fields in an edit mode panel that I wanted to become visible when the user was editing the page. For some reason, SharePoint was not able to detect that these pages were editable.
To resolve the issue, I included a hidden web part zone on the layout page. Even though it is hidden, SharePoint thinks that the page should now be editable.