Hi,
I'm having a the same original issue as dotbeta, with the difference that I have not setup the wildcard DNS entry because Hostmonster won't allow it.
I'm trying to use a secondary domain mapped to a sub-blog, but when I try to login to it, I get the exact same issue, it loads the login page again.
I don't know why it works, but accessing the secondary domain opens the blog perfectly well, I just can't login to it. I parked my secondary domain to my main domain, setup the new blog, and changed the subdomain address options to point to the new domain (which I know should not be done). My Domain Mapping plugin is actually disabled. If I try to access the blogs subdomain (on my main domain), it won't load.
So the "mapping" is working correctly, since if I try to access blog2 at http://www.blog2.com, it works perfectly for the user, and you never know that it is running at mainblog.com, but an admin user for blog2 cannot login.
Do you know if there is any way that I can get this working without the wildcard DNS entry? Thanks a lot in advance!