On a test site running WordPress 3.4-RC2-21036, I updated my own plugin (which is in the repository) to the latest version and it generated a fatal error to the effect that it could not find the main .php file. Doing the same plugin update on WordPress 3.3.2 works perfectly.
Unfortunately, I thought it was my plugin at fault so did not capture the exact error message, and I don't know how to reproduce the problem.
Here is my best summary of what is wrong with 3.4RC2: automatic update of a plugin where the main .php file has been renamed from one plugin version to the next, will generate a fatal error on activation.
Workaround: uninstall and reinstall the plugin.
Plugin in question: http://wordpress.org/extend/plugins/jonradio-display-kitchen-sink/
Upgrade from 1.0 to 1.1 corrects the name to jonradio-display-kitchen-sink/jonradio-display-kitchen-sink.php from jonradio-display-kitchen-sink/jonradio-kitchen-sink.php