Hi there,
i'm having the exact same problem with the latest version 3.2.1 (First install, default installation, no plugins, followed the steps in install guide), WP_DEBUG wont give me any useful info and yes, i already spent all day looking on the internet for "wordpress login loop".
After a bit of digging in the source code, TamperData and Firebug i found out that i am redirected from wp_login.php to /wp-admin/ and then back to wp_login.php again without any error message or useful information.
As far as i can see it, the problem exists in /wp-includes/pluggable.php at function auth_redirect(), somewhere around line 824:
if ( $user_id = wp_validate_auth_cookie( '', $scheme) ) {
What is this? $user_id has not been defined or declared global inside this function (or am i missing something here?), hence it always skips this block and continues further down at // The cookie is no good so force login, kicking me back to wp-login.php and the the loop starts again.
I can confirm that 2 cookies are set: wordpress_test_cookie = WP+Cookie+check and PHPSESSID = 161293d38543c1745381e6d939b957ee, and there is an empty (0 Bytes) session file sess_161293d38543c1745381e6d939b957ee in my php session folder, and the cookie-host is equal to whats set in tbl wp-options under siteurl and home.
Any ideas?