Anti-Splog has a terrific feature called 'Rename wp-signup.php' that changes the signup URL every 24hrs to prevent sploggers.
Unfortunately, if this changeover occurs while a user is in the middle of the signup process, they get a 404 error when continuing to the next page.
Please implement a temporary redirect/overlap that will make the previous signup URL remain active or redirect to the current URL for a short period of time (i.e. 15min, 1hr, whatever).
It'd be ideal if this redirection/overlap was transparent to the user, i.e. they didn't lose any of their information or have to restart.