I had been using wordpress 2.3. I downloaded version 2.3.1, and uploaded it to my site. I am now getting this dialogue box, and cannot view my blog at all.
It is asking for username and password, for an un-named user. I checked with my hosting control panel, and the SQL username is beretta. Since I had just backed up my blog in version 2.3, I could not be bothered fiddling around with the new one, so I just deleted my blog directory from the server and uploaded the old one. Problem fixed, right? No! That dialogue box still shows up. It has never appeared before.
The file wp-config-sample.php has all the correct username and password details in it, yet a fresh install of either version of wordpress - with that correct info included, is not doing anything to fox the problem.
I've upgraded several times before and this has never occurred. I wish I had not upgraded this time. Now nobody can even see my blog at http://www.beretta-online.com/wordpress , not even me!