First of all, I am sorry if this issue has be handled already, I couldn't find it by searching/looking manually.
I just upgraded from 2.5 to 2.5.1, was prompted to upgrade, which went fine -- it completed upgrade process.
(I even deleted all WP files and copied them all over again, but it still isn't working.)
But after upgrade I can't access index/dashboard page in admin section (wp-admin/index.php) and sub pages that index.php produces.
Everything else works fine, version has been bumped also.
Sometimes I can access admin index/dashboard, but only if I leave browser to load the page for minutes at the time. It seems like some of the external links (feeds) are not accessible by servers (yes, tried on several different servers at different locations, so it shouldn't be network).
Also, two minutes before upgrade, dashboard worked just find in WP 2.5. Now, 8-9 hours after upgrade, it's still not working, so I am pretty confident that it isn't network issue.
Done some, "low-tech" debugging, and found that it hangs on an if-statement, in wp-admin/menu-header.php
<?php // Sub-menu if ( isset($submenu["$parent_file"]) ) : ?>
$parent_file holds "index.php" value.
while $submenu, does not have key named "index.php".