This may be fixed by the next release, but I wanted to mention it anyway just in case. If a user has logged, and later the admin changes that user's group setting (to say a moderator), when the member attempts to log back into the site, they will not be able to log in until they click on the clear cookies link.
I have only tested this using the login directly from the Member API. I have disabled all other login forms. I have been able to duplicate this every time.
Thanks,
Brian
WWF 9.03
IE 7 and FF 2