The problem is that a user can't progress through the Registration wizard because when they click Continue they get taken back the login page instead.
Each time this user's browser sends a POST it immediately follows with a GET to /, which the server redirects to the welcome page (and then to the login page because of course they are not logged in).
xxx.69.247.zzz - - [19/Jun/2010:20:53:30 +1200] "POST /...
xxx.69.247.zzz - - [19/Jun/2010:20:53:32 +1200] "GET / HTTP/1.1" 302 190 2397 "-"
This is caused by Content Advisor in IE.
It seems to affect all our unauthenticated forms - if you have Content Advisor enabled, you cannot proceed past the first page because of the extra GET request.
Curiously, authenticated forms are not affected - the extra GET is still sent but the forms behave correctly.
There is a known bug in IE 6 (http://support.microsoft.com/default.aspx?scid=kb;EN-US;924456). The user below was using IE 8 so likely something similar in other versions of IE.
The servers don't have a content rating, but even if you tell Content Advisor to approve those sites the GET requests still are sent (ie. adding a content rating probably wouldn't fix this).
