I ran into both the 403 Forbidden error and the one included as the topic of this post. The 403 error I fixed by adding the line:
DirectoryIndex index.html index.php
Witihin the VirtualHost defined in my local.conf (a.k.a httpd.conf) file for Apache. W/o it Apache didn't know to open up /wp-admin/index.php when wp-login.php redirected to /wp-admin/ upon logging in as the administrator.
I fixed the "You do not have sufficient permissions to access this page." error by changing the following line in my PHP.ini:
magic_quotes_runtime = Off
It was set to "On", and was therefore escaping quotes from data taken from the database. The code for wordpress can probably be changed so this directive can remain "On" if you'd rather be more secure, I suppose. That's just a hunch.
Anyways, these two small changes worked for me. Just thought I'd share.