I've upgraded to 3.0 today without any trouble. I created a new site, the site appeared in the list of sites but I checked the files via FTP only to find that nothing was actually created.
I tried to visit the site I just created and all I saw was just a blank page. In the Admin page it still says 'You have 1 site' although there should be 2 sites.
The first site I created before uploading works perfect.
Any clues?? Thanks.