Honestly, what good will XCODE do, if you can't also use a terminal and install command line tools, libraries, ruby gems, etc... I don't see Apple ever opening up an iPAD like that. And at that point, why wouldn't you just want a MacBook Pro instead?

Seems like a lot of things can trigger this error. I just solved it for me (I'm using a free developer account) by Looking at XCODE->Settings->Accounts->Manage Certificates, and I saw a revoked certificate in the list. I deleted that, and my build succeeded.