Since, it took me over an hour to diagnose and fix this issue in one of my own WordPress installations, I thought I would write up a quick how-to on the solution to this issue to hopefully save others in wasting their own time.
Accessing the Edit screen via WordPress Admin showed a blank screen in several versions of WordPress 2.8. The rest of the Admin was working fine.
I use Subversion to automate the update/upgrade to new versions of WordPress. This has worked great for me for awhile without incident. In the most recent update, the post.php file was updated and the Subversion update command changed the ownership of the file to the user root, which my script was running as.
This permission changed caused a subtle PHP error, and since the recent version of PHP 5 on my server has the default behavior of not displaying errors – a blank white screen was being shown in WordPress.
Reset the permissions recursively on your entire WordPress install.
For example, if your user and group were joesmith and users, run the following command at the root of your WordPress install folder:
chown -R joesmith:users *
You should then be back in business!
So the moral of this story is, if you update your WordPress blog via Subversion – fix your file permissions afterwards.