I like the staying dead theory, but I also really like the Doctor theory. I mean, that would make perfect sense. The Doctor's always bumming around Earth, so at some point the TARDIS is sure to pick up on that signal from the ring. And then the Doctor would be like, hm, that doesn't belong in 27 CE, let's take a look. And then he'd dig up Jack and be like, oh, I should've known it was you. If you wanted a date, you could've just called.