I just installed a fresh copy of 3.0.4, went through the steps to create a multisite and everything looked good. Then, I attempted to sign in to one of the newly created Sites and the login page simply repeats itself. No errors, not even in the log. The main install works fine, this seems to be isolated just to new sites.
I’ve tried logging in using the Super Admin user and the Site Admin user, both do the same thing. Anyone seen this before?
I am guessing that you are using subdomains instead of sub folders for your multisite set up? This is an issue that appears to happen when using subdomains and setting up your multisite manually.
I recall this issue happening with WordPress MU a bit ago and it was related to cookies or something and you simply had to add some cookie settings to your config file. Because multisites are WordPress MU basically, I wonder if that’s the issue.
Try this comment posted on the WordPress support forums. This question on WordPress answers looks similar, does this help as well?