I have two installations of WP-2.0.2. In one of them the tinyMCE editor appears and in the second one it does not.
I verified that the wp-includes directory is present and readable. I used the same Firefox browser to test each one, and I verified that the checkbox in my profile for using the rich editor was checked.
I debugged and solved the problem as follows:
Call to undefined function: file_get_contents() in /var/www/html/wp-includes/js/tinymce/tiny_mce_gzip.php on line 67.
That error led me to discover that PHP versions older than 4.3 do not support file_get_contents(). I checked the version running on the failing instance and it was 4.2.2.
Every other function of WordPress was working fine. Only the rich editor was failing to load. It could be fixed by upgrading PHP, but in case that's not possible, it can be worked-around by adding the following near the top of tiny_mce_gzip.php:
That library (by including functions-compat.php) contains a compatibility function to replace file_get_contents().