Hi. I'm having a similar issue. This is the message in the error log:
[17-Jul-2010 16:23:48] PHP Fatal error: require() [<a href='function.require'>function.require</a>]: Failed opening required '/home2/marshall/public_html/sargemarcori/wordpress/wp-includes/default-constants.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home2/marshall/public_html/sargemarcori/wordpress/wp-settings.php on line 20
I manually upgraded to 3.0 and everything was fine. I'd run the site upgrade and logged in, but then I tried to fix that fatal memory error by getting rid of the Super Cache plug-in because nothing else was working to get rid of it. I messed things up by deleting the plug-in through FTP when I couldn't get to it in the admin. Since then both my admin and site at http://www.sargemarcori.com have been blank. I tried uploading the file in the error message again, and when that didn't work I tried deleting wp-admin and wp-includes and uploading them again.
I did try it with a fresh download of 3.0. I'm using Filezilla 3.3.3 if that matters, on Vista OS. I can get to the ftp information by going to ftp:// in my browser, but I don't know what to look for to figure out what I screwed up.
Also though I have tried to go to the site several times today, the last entry in the error log is over five hours old.
Any help would be hugely appreciated!