I enabled random register page in anti splog and I had to edit a couple of direct links in my theme and change to ust_wpsignup_url() . I checked that wp-login.php?action=register works on the main blog which it does.
on sub blogs though, the sublog.domain.com/wp-login.php?action=register link that shows in default sidebar or meta widget will forward pages until it ends up on the wp-signup.php on the main blog which of course shows the location changed error.
everything else is working fine.
(I have wp-supercache running too, do you think it could be affecting how the links and page work?)
edit [sorry for dupe post, I got a page not available so clicked back and submitted again]