Is it possible that Miller is protecting herself because she is the original leak???
It's possible that Miller is the one who told Karl Rove (though I suspect it was Tennant).
It is also possible that Plame (or Wilson) was Miller's source. Miller is a reporter specializing in WMD. Plame was working for CIA, specializing in WMD. It's quite likely they knew each other -- and what each other did.
Given that Miller won't answer questions about a source for a story she never wrote, it's possible that she became involved in the investigation because somebody (Rove?) said "Miller is the one who told me about Plame".