Tumblr Beta
tumblr-beta.com
tumblr-beta.com
edit: apparently not: http://staff.tumblr.com/post/45588160/tumblr-4-beta
I understand if you need to run the site on another domain (because of cookies) but please don't train your users to be phished. The login page should have been on tumblr.com/beta (or beta.tumblr.com) that posts to the other site (or if you have an authenticity token to protect against CSRF posts to the login path, you could iframe the login page from tumblr-beta.com/login inside a tumblr.com domain).