For anyone who is seeing this problem that the Visual Editor is not appearing, I have seen this as well on a number of installs. The cuplrit appears to be with the way that the TinyMCE editor is using gzip compression. I found this bug which reported a similar issue:
http://trac.wordpress.org/ticket/3144
And the resolution here:
http://trac.wordpress.org/attachment/ticket/3144/3144-gzipoptoins-tinymce.patch
worked for me. In the file wp-includes/js/tinymce/tiny_mce_gzip.php I changed the line:
if ((in_array('gzip', $encodings) || in_array('x-gzip', $encodings) || isset($_SERVER['---------------'])) && function_exists('ob_gzhandler') && !ini_get('zlib.output_compression')) {
to read this:
if ((in_array('gzip', $encodings) || in_array('x-gzip', $encodings) || isset($_SERVER['---------------'])) && function_exists('ob_gzhandler') && !ini_get('zlib.output_compression') && get_settings('gzipcompression')) {
and that immediately solved the problem: the Visual Editor appeared just as it should.