I’m having the same trouble. Mine started a couple of days ago. My wife was in the middle of typing a post, was logged out and, since, we’ve been unable to get beyond wp-login.php. It’ll recognize an incorrect login, but a correct login returns you to the username/password prompt as well. I’ve, since, upgraded to WP 2.6.3, but have seen no change. Coincidentally, my hosting provider is Dreamhost, also.