I’ve been periodically getting a “Redirect Loop” error in WordPress. I wasn’t sure what the source was. I tried the things you are supposed to do (deleting cookies, especially), and no luck. I went in via FTP and temporarily swapped all my plugins out of the plugins directory. This let me log in, which suggests that one of the many plugins is the culprit. No, I don’t know which one. I’ll try adding them in again one-by-one to track it down.
For now, if you are having a frustrating Redirect Loop problem, look to the plugins. That is all.