I'm finding this problem to be pretty widely reported. I run wordpress multisite, which I think has a problem. I think that problem is described here. http://tommcfarlin.com/resolving-the-wordpress-multisite-redirect-loop/
Specifically, the problem is this:
Install WordPress and activate multisite
Configure the installation to use subdomains
Attempt to login and get stuck in a redirect loop
This is exactly what happened to me, on a 100% fresh wordpress install. I tried it several times, and after activating multisite the logins broke every time.
The fix they provide works for me, it involves adding the following to wp-config.php
The problem I'm finding is that installing WordPress MU Domain Mapping, conflicts with the fix above, because it makes you comment out, or "remove define('COOKIE_DOMAIN', '');" - which in turn causes the redirect loop to come back.