MacBuildServer is distributing the .ipa files they have inhouse signed to anyone that wants it. Apple would only revoke the certs used to sign apps that are distributed outside that assigned enterprise.
Okay, that I could see happening, and I wouldn't have much of a problem with it (abusing the enterprise keys and all). Perhaps I misread, as I took it to be asking about the general case of any dev who downloads source and clicks "Product/Run".
