Why in Xcode 6, I am getting "You are not registered as an Apple developer" while trying to upload a binary?

The following fixed it for me:

In Xcode > Preferences > Accounts, I had an Apple ID account that wasn't a registered development, in addition to my own valid account. I deleted the unregistered Apple ID, and it now works.

I had almost the same situation as @iht, except that the "unregistered" Apple ID was a legit-but-new developer ID.

Turns out I hadn't "signed" the dev agreement yet, so I logged into developer.apple.com, was presented with the wall of fine print, clicked Agree and was set to go.