Apple has messed up their developer certificate stuff. The actual certificates work ok, but the provisioning profiles (which are used when testing applications on devices and when sending the application to the AppStore) won't work. Seems that Apple has forgotten to put the certificate information into the provisioning files. And many people are pissed. Fortunately I didn't need my provisioning profiles right now even though I ran into this problem.
Nice work, Apple. Keep it up. It's not ok to mess up stuff for everyone just because you're releasing iPhone OS 3.0 today...
More info about the identity matching provisioning profile mess.