I’m having the same problem. I can only access the HTML editor, clicking on “Visual” does nothing. I have a white page with white text so I can’t see as I’m typing, can only see if I highlight the text.
I used WordPress Automatic Upgrade 1.2.5 to perform the upgrade, and it looks like I’m not the only one. Perhaps there is a link between the two, and maybe a way we can fix whatever mistake was made in the upgrade so I don’t have to re-install WordPress 2.8 and reload all of my plugins. I think for now I’ll go back to 2.7.1 using the backup I made.
Thanks for your help!