Creating a new ticket as per evands comment on ticket 9836.
I have tried the language change trick, and I know for certain that I enter my password correctly (checked on the site just to make absolutely sure). It just keeps prompting for my password, as it doesn't authenticate.
The login page contains the error "Incorrect email/password combination." As I said, I know I entered it correctly, and if I change my password so it no longer contains an ampersand, it authenticates just fine. Guessing the password just didn't get application/x-www-form urlencoded prior to sending.
Just in case, I've attached the debug messages (with password snippet replaced with asterisks). Also, not sure if it changes anything, but I have Safari 4 Beta (Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_5_3; en-us) AppleWebKit/526.11 (KHTML, like Gecko) Version/4.0dp1 Safari/526.11.2) installed.