I got the same problem here, the visual editor just went away after upgrading from 2.7.1 to 2.8 (thankfully this is not a production or even public site).
I’ve done some reseach with firebug and the problem seems to be related with the version that is requested from wp-includes/js/tinymce/wp-tinymce.php , in my case, looking at the network traffic I see this request:
wp-includes/js/tinymce/wp-tinymce.php?c=1&ver=3241-1141
which is returning nothing, but if I change that to be
wp-includes/js/tinymce/wp-tinymce.php?c=2&ver=3241-1141
then it returns the needed javascript for tinymce.
I looked at the source for wp-admin/post-new.php and it’s using the wp_enqueue_script function to load the ‘editor’ scripts.
Digging a bit, that function is defined on wp-includes/functions.wp-scripts.php….
And there’s where I’m right now, I’ll let you know as soon as I know more.